See this page as a slide show

Introduction to TCP/IP Networking

Thanks to:

• Dr. Indrajit Ray, CSU
• Dr. James Walden, NKU
• Russ Wakefield, CSU

for the contents of these slides.

Definition: host

A host is a thing on a network; anything with an IP address. My house contains these hosts:

Definition: network

What is a network? Hosts connected by a single medium.

    ┌────────┐                    ┌────────┐
    │        │                    │        │
    │ laptop │····················│  NAS   │
    │        │      ethernet      │        │
    └────────┘       cable        └────────┘

Media include cat-5, Wi-Fi, cable-tv cable, fiber-optics, infrared light, phone lines, power lines, short-wave radio, lasers, sub-space pulses, etc. Pick one! If you have more than one, you have several networks.

Singular: “medium”
Plural: “media”

Definition: LAN

A LAN is a Local Area Network; a small-scale network. I have two LANs in my house: wired & wireless.

LAN ≠ Ethernet!

Variations on a theme

• BAN: Body Area Network
• CAN: Campus Area Network
• HAN: Home Area Network
• LAN: Local Area Network
• MAN: Metropolitian Area Network
• NAN: Near-me Area Network
• PAN: Personal Area Network
• SAN: Storage Area Network
• WAN: Wide Area Network

Definition: internet

What is a (lower-case “i”) internet(work)? It’s a collection of connected networks.

    ┌────────┐           ┌────────┐              ┌─────────┐
    │        │           │        │              │         │
    │ laptop │···········│ router │··············│ printer │
    │        │   wi-fi   │        │   ethernet   │         │
    └────────┘           └────────┘              └─────────┘

I have an internet at home (wi-fi + Ethernet).

Definition: Internet

What is the (upper-case “I”) Internet? It’s all the connected networks in the world. There are many internets, but only one Internet.

    ┌────────┐           ┌────────┐              ┌──────┐
    │        │           │        │              │      │
    │ laptop │···········│ router │··············│ eBay │
    │        │   wi-fi   │        │   Internet   │      │
    └────────┘           └────────┘              └──────┘

or, more simply:

    ┌────────┐              ┌──────┐
    │        │              │      │
    │ laptop │··············│ eBay │
    │        │   Internet   │      │
    └────────┘              └──────┘

Definition: World-Wide-Web

Tim Berners-Lee

What is the (World-Wide-)Web? It’s a use of the Internet.

• The Web is built on top of the internet.
• My personality is built on top of my brain.

Definition: more terms

    ┌────────┐              ┌──────┐
    │ laptop │··············│ eBay │
    └────────┘   Internet   └──────┘

Consider a web browser, on your laptop, connected to ebay.com.

The laptop and the web server are both end systems, or hosts.

End systems can also include printers, surveillance cameras, cell phones, and generally any device using the network to communicate.

End systems are located at the network edge and connected to the network using communication links.

Definition: Client/Server

    ┌────────┐              ┌──────┐
    │ laptop │··············│ eBay │
    └────────┘   Internet   └──────┘

End systems may be classified as clients, servers, both, or neither.

• Client: a program that requests services:
  • web browser requests a page, email reader requests messages,
  • scp program requests files, etc.
• Server: a program that listens for requests and provides services
  • web server, email server, ssh server, etc.

Client vs. server depends on what programs the end system is running. A given host may be both. Most hosts are DNS clients.

Sending a packet

┌─────────────────┐       ┌───────────────┐       ┌───────────────┐
│     ct320-1     │  lan1 │    router     │       │     boise     │
│ 192.168.110.101 │·······│ 192.168.110.1 │   ····│ 129.82.46.197 │
└─────────────────┘   :   ├ ─ ─ ─ ─ ─ ─ ─ ┤   :   └───────────────┘
┌─────────────────┐   :   │    acushla    │   :   ┌───────────────┐
│     ct320-2     │   :   │ 129.82.47.50  │·······│     salem     │
│ 192.168.110.102 │····   └───────────────┘  lan2 │ 129.82.46.233 │
└─────────────────┘                               └───────────────┘

• There are two networks (LANs) present:
  • On the left, lan1, 192.168.110.0/24, in the CSB 315 lab.
    • This is part of 192.168.0.0/16, reserved for private networks.
  • On the right, lan2, 129.82.44.0/22, the rest of the building.
    • This is part of 129.82.0.0/16, allocated for CSU.
• The center box is a router, connected to both LANs.

Sending a packet

┌─────────────────┐       ┌───────────────┐       ┌───────────────┐
│     ct320-1     │  lan1 │    router     │       │     boise     │
│ 192.168.110.101 │·······│ 192.168.110.1 │   ····│ 129.82.46.197 │
└─────────────────┘   :   ├ ─ ─ ─ ─ ─ ─ ─ ┤   :   └───────────────┘
┌─────────────────┐   :   │    acushla    │   :   ┌───────────────┐
│     ct320-2     │   :   │ 129.82.47.50  │·······│     salem     │
│ 192.168.110.102 │····   └───────────────┘  lan2 │ 129.82.46.233 │
└─────────────────┘                               └───────────────┘

• Our goal is to send a packet from ct320-1 to salem.
• How does that work?

Sending a packet

┌─────────────────┐       ┌───────────────┐       ┌───────────────┐
│     ct320-1     │  lan1 │    router     │       │     boise     │
│ 192.168.110.101 │·······│ 192.168.110.1 │   ····│ 129.82.46.197 │
└─────────────────┘   :   ├ ─ ─ ─ ─ ─ ─ ─ ┤   :   └───────────────┘
┌─────────────────┐   :   │    acushla    │   :   ┌───────────────┐
│     ct320-2     │   :   │ 129.82.47.50  │·······│     salem     │
│ 192.168.110.102 │····   └───────────────┘  lan2 │ 129.82.46.233 │
└─────────────────┘                               └───────────────┘

One does not simply resolve a name with DNS

• ct320-1: DNS: What is the IP address of salem?
• DNS replies that salem is 129.82.46.233
• OK, truly, there was a lot of networking that we ignored there.

Sending a packet

┌─────────────────┐       ┌───────────────┐       ┌───────────────┐
│     ct320-1     │  lan1 │    router     │       │     boise     │
│ 192.168.110.101 │·······│ 192.168.110.1 │   ····│ 129.82.46.197 │
└─────────────────┘   :   ├ ─ ─ ─ ─ ─ ─ ─ ┤   :   └───────────────┘
┌─────────────────┐   :   │    acushla    │   :   ┌───────────────┐
│     ct320-2     │   :   │ 129.82.47.50  │·······│     salem     │
│ 192.168.110.102 │····   └───────────────┘  lan2 │ 129.82.46.233 │
└─────────────────┘                               └───────────────┘

• ct320-1: 129.82.46.233’s not on my local network, therefore I need to send the packet to my default gateway, 192.168.110.1.
• A gateway is where to send stuff when you don’t know where it should go. It’s assumed that the gateway knows more than you do, or, that it has even smarter friends.

Sending a packet

┌─────────────────┐       ┌───────────────┐       ┌───────────────┐
│     ct320-1     │  lan1 │    router     │       │     boise     │
│ 192.168.110.101 │·······│ 192.168.110.1 │   ····│ 129.82.46.197 │
└─────────────────┘   :   ├ ─ ─ ─ ─ ─ ─ ─ ┤   :   └───────────────┘
┌─────────────────┐   :   │    acushla    │   :   ┌───────────────┐
│     ct320-2     │   :   │ 129.82.47.50  │·······│     salem     │
│ 192.168.110.102 │····   └───────────────┘  lan2 │ 129.82.46.233 │
└─────────────────┘                               └───────────────┘

• ct320-1: I need to send the packet to 192.168.110.1.
• Good, that’s on my LAN.
• I need a MAC address for this, not an IP address.

Sending a packet

┌─────────────────┐       ┌───────────────┐       ┌───────────────┐
│     ct320-1     │  lan1 │    router     │       │     boise     │
│ 192.168.110.101 │·······│ 192.168.110.1 │   ····│ 129.82.46.197 │
└─────────────────┘   :   ├ ─ ─ ─ ─ ─ ─ ─ ┤   :   └───────────────┘
┌─────────────────┐   :   │    acushla    │   :   ┌───────────────┐
│     ct320-2     │   :   │ 129.82.47.50  │·······│     salem     │
│ 192.168.110.102 │····   └───────────────┘  lan2 │ 129.82.46.233 │
└─────────────────┘                               └───────────────┘

• ct320-1: ARP: What is the MAC address of 192.168.110.1?
• ARP: The MAC addr of 192.168.110.1 is 00:e0:f9:23:a8:20
• ct320-1: puts a packet on its LAN addressed to 00:e0:f9:23:a8:20

Sending a packet

┌─────────────────┐       ┌───────────────┐       ┌───────────────┐
│     ct320-1     │  lan1 │    router     │       │     boise     │
│ 192.168.110.101 │·······│ 192.168.110.1 │   ····│ 129.82.46.197 │
└─────────────────┘   :   ├ ─ ─ ─ ─ ─ ─ ─ ┤   :   └───────────────┘
┌─────────────────┐   :   │    acushla    │   :   ┌───────────────┐
│     ct320-2     │   :   │ 129.82.47.50  │·······│     salem     │
│ 192.168.110.102 │····   └───────────────┘  lan2 │ 129.82.46.233 │
└─────────────────┘                               └───────────────┘

• Everybody on lan1 receives the packet; most ignore it.
• Router: receives the packet on its lan1 port.
• Router: looks at the IP addr in the packet, 129.82.46.233.
• Router: That’s on my (other) local network, so I can send that directly.

Sending a packet

┌─────────────────┐       ┌───────────────┐       ┌───────────────┐
│     ct320-1     │  lan1 │    router     │       │     boise     │
│ 192.168.110.101 │·······│ 192.168.110.1 │   ····│ 129.82.46.197 │
└─────────────────┘   :   ├ ─ ─ ─ ─ ─ ─ ─ ┤   :   └───────────────┘
┌─────────────────┐   :   │    acushla    │   :   ┌───────────────┐
│     ct320-2     │   :   │ 129.82.47.50  │·······│     salem     │
│ 192.168.110.102 │····   └───────────────┘  lan2 │ 129.82.46.233 │
└─────────────────┘                               └───────────────┘

• Router: I need to send this packet to 129.82.46.233.
• Router: ARP: What’s the MAC addr of 129.82.46.233?
• ARP: 129.82.46.233 is 00:20:af:03:98:28
• Router: sends a frame on the Ethernet connected to its lan2 port to 00:20:af:03:98:28

Sending a packet

┌─────────────────┐       ┌───────────────┐       ┌───────────────┐
│     ct320-1     │  lan1 │    router     │       │     boise     │
│ 192.168.110.101 │·······│ 192.168.110.1 │   ····│ 129.82.46.197 │
└─────────────────┘   :   ├ ─ ─ ─ ─ ─ ─ ─ ┤   :   └───────────────┘
┌─────────────────┐   :   │    acushla    │   :   ┌───────────────┐
│     ct320-2     │   :   │ 129.82.47.50  │·······│     salem     │
│ 192.168.110.102 │····   └───────────────┘  lan2 │ 129.82.46.233 │
└─────────────────┘                               └───────────────┘

• Everyone on lan2 receives the packet; most ignore it.
• salem: Hey, that’s my MAC addr! That packet must be for me!

Sending a packet

┌─────────────────┐       ┌───────────────┐       ┌───────────────┐
│     ct320-1     │  lan1 │    router     │       │     boise     │
│ 192.168.110.101 │·······│ 192.168.110.1 │   ····│ 129.82.46.197 │
└─────────────────┘   :   ├ ─ ─ ─ ─ ─ ─ ─ ┤   :   └───────────────┘
┌─────────────────┐   :   │    acushla    │   :   ┌───────────────┐
│     ct320-2     │   :   │ 129.82.47.50  │·······│     salem     │
│ 192.168.110.102 │····   └───────────────┘  lan2 │ 129.82.46.233 │
└─────────────────┘                               └───────────────┘

• That was a ridiculous amount of work.
• In practice, ct320-1 would have remembered the IP address of salem from a previous interaction. This is called caching.
• Also, ct320-1 would have cached the MAC address of the router.
• Similarly, the router would have cached the MAC address of 129.82.46.233.

Communications Architecture

• The complexity of the communication task is reduced by using multiple protocol layers:
  • Each protocol is implemented independently
  • Each protocol is responsible for a specific subtask
  • Protocols are grouped in a hierarchy
• A structured set of protocols is called a communications architecture or protocol suite

Divide & Conquer

• It’s all about divide & conquer, or delegation.
• Don’t try to do everything at once.
• Break a big problem apart into smaller sub-problems.
• Networking is enormously difficult, so we break it into layers.

Layers Example

• Back at HP, my boss would trust me to write the program that he wanted.
• I trust the compiler to accurately translate my C code to machine language instructions.
• I trust the cpu to accurately decode & execute instructions.
• The cpu assumes that it will have reliable DC power from the power supply.
• The power supply assumes that the outlet will supply 120VAC.
• and so on

Each layer assumes that the layer below it will do its job.

TCP/IP Protocol Suite

• The TCP/IP protocol suite is the protocol architecture of the Internet
• The TCP/IP suite has four layers:
  • Application
  • Transport
  • Network
  • Data Link
• Mnemonic aids:
  • attend (pay attention)
  • atoned (they should have atoned for this)
• End systems (hosts) implement all four layers.
• Gateways (Routers) only have the bottom two layers.

The Layers

Functions of the Layers

• Application Layer:
  • Service: Application specific (delivery of email, retrieval of HTML documents, reliable transfer of file)
  • Functions: Application specific
• Transport Layer:
  • Service: Delivery of data between hosts
  • Functions: Connection establishment/termination, error control, flow control
• Network Layer:
  • Service: Move packets from source host to destination host
  • Functions: Routing, addressing
• Data Link Layer:
  • Service: Reliable transfer of frames over a physical medium
  • Functions: Framing, media access control, error checking

Assignment of Protocols to Layers

• Application Layer:
  • ping application, HTTP, SSH, DNS, SNMP, SMTP, DHCP
• Transport Layer:
  • TCP, UDP
• Network Layer:
  • IP, ICMP
• Data Link Layer:
  • ARP, Ethernet

Layered Communications

• An entity of a particular layer can only communicate with:
  1. a peer layer entity using a common protocol (Peer Protocol)
  2. adjacent layers to provide services and to receive

┌──────────────────┐   Layer N+1 protocol  ┌──────────────────┐
│ Layer N+1 entity │<–––––––––––––––––––––>│ Layer N+1 entity │
└──────────────────┘                       └──────────────────┘
  │             ∧                             │             ∧
  ∨             │                             ∨             │
┌──────────────────┐   Layer N protocol    ┌──────────────────┐
│ Layer N entity   │<–––––––––––––––––––––>│ Layer N entity   │
└──────────────────┘                       └──────────────────┘
  │             ∧                             │             ∧
  ∨             │                             ∨             │
┌──────────────────┐   Layer N-1 protocol  ┌──────────────────┐
│ Layer N-1 entity │<–––––––––––––––––––––>│ Layer N-1 entity │
└──────────────────┘                       └──────────────────┘

Layered Communications

A layer N+1 entity sees the lower layers only as a service provider:

┌──────────────────┐   Layer N+1 protocol  ┌──────────────────┐
│ Layer N+1 entity │<–––––––––––––––––––––>│ Layer N+1 entity │
└──────────────────┘                       └──────────────────┘
  │                                                         ∧
  │                                                         │
  │request                                          indicate│
  │delivery                                         delivery│
  │                                                         │
  ∨                                                         │
┌─────────────────────────────────────────────────────────────┐
│                    Service provider                         │
└─────────────────────────────────────────────────────────────┘

Application View of Networking

Application job: write the web browser (client) or web server (server)

    ┌─────────┐           ┌──────────┐           ┌──────┐
    │ laptop  │           │          │           │      │
    │ running │···········│ Internet │···········│ eBay │
    │ browser │           │          │           │      │
    └─────────┘           └──────────┘           └──────┘

Assume network provides way to send a messages between hosts.

• Don’t know or care how the messages are sent.
• Do care about:
  • Does the network provide a connection or is it connectionless?
  • Are messages reliable?
  • Who/what provides flow control? (speed of sending messages)

Transport View of Networking

Transport job: implement the connection-(oriented/less) service

    ┌─────────┐           ┌──────────┐           ┌──────┐
    │ laptop  │           │          │           │      │
    │ running │···········│ Internet │···········│ eBay │
    │ browser │           │          │           │      │
    └─────────┘           └──────────┘           └──────┘

• Assume application handles message content.
• Don’t know or care about the content of the messages.
• Don’t know or care how the messages get from source to destination
• Do care about:
  • How to provide a connection or connectionless service?
  • How to make the transport connection reliable?
  • How to handle congestion and flow control in the network?

Network Layer View

Network layer job: get a message from a source to a destination

    ┌─────────┐
    │ laptop  │     ┌───────┐    ┌───────┐    ┌───────┐
    │ running │·····│ ISP A │····│ ISP B │····│ ISP C │
    │ browser │     └───────┘    └───────┘    └───────┘
    └─────────┘         :            :            :
        	        :            :            :
                    ┌───────┐    ┌───────┐    ┌───────┐      ┌──────┐
                    │ ISP D │····│ ISP E │····│ ISP F │······│ eBay │
                    └───────┘    └───────┘    └───────┘      └──────┘

• Assume higher layers handle message content, reliability.
• Care about:
  • How to provide a best effort attempt to deliver packets?
  • Routing: A→B→C→F→eBay, A→D→E→F→eBay, A→B→E→F→eBay, or what?

Link Layer View of Networking

Link Layer job: get a message sent across some medium

    ┌────────┐                             ┌────────┐
    │ host A │·····························│ host B │
    └────────┘      direct connection      └────────┘

• Only care about how to get message from A to B across this link.
• Link can be twisted pair, coaxial, fiber optic, radio, infrared, etc.
• Has to deal with messy physical stuff:
  • voltage levels
  • radio signals
  • noisy connections

Exchange of Data

• The unit of data sent between peer entities is called a Protocol Data Unit (PDU)
• For now, let us think of a PDU as a single packet

        ┌─────────┐           ┌───────────┐           ┌─────────┐
      A │ Layer N │           │    PDU    │         B │ Layer N │
        │ entity  │···········│ (layer N) │···········│ entity  │
        └─────────┘           └───────────┘           └─────────┘

• Scenario: Layer-N at A sends a layer-N PDU to layer-N at B
• What actually happens:
  • A’s layer-N passes the PDU to layer-N-1
  • Layer-N-1 entity at A constructs its own (layer-N-1) PDU which it sends to the layer-N-1 entity at B
  • PDU at layer-N-1 = layer-N-1 Header + layer-N PDU

Love Letters

My wife is visiting her family in France, and I miss her dearly, so I write a passionate love letter:

• My wife cares only about this three-line message.
• Can I just hand this to the postman? No, he wants an envelope!
• The postman doesn’t care about the love letter—he only cares about the envelope.
• Is that the only wrapper?

Example

Say that a web browser wants to see eBay’s home page. It might construct this 34-byte HTTP request:

However, to send this over a TCP connection, the Transport level adds a 20-byte header, so now we have this:

Example

The Transport level hands this off to the Network level, which will create an IP packet with its own 20-byte header:

The Network level hands this off to the Data Link layer, which will add its own overhead, depending on the medium:

Example

When sending, each level treats the data from the previous level as a black box, an unintelligible blob of stuff, and adds its own header or footer.

Similarly, when receiving, each level removes its overhead.

Layers in the Example

┌──────────┐                                        ┌──────────┐
│   HTTP   │                                        │   HTTP   │
└──────────┘                                        └──────────┘
     │                                                   │ 
┌──────────┐                                        ┌──────────┐
│   TCP    │                                        │   TCP    │
└──────────┘                                        └──────────┘
     │                                                   │ 
┌──────────┐              ┌──────────┐              ┌──────────┐
│   IP     │              │    IP    │              │   IP     │
└──────────┘              └──────────┘              └──────────┘
     │                      │      │                     │ 
┌──────────┐      ┌──────────┐    ┌──────────┐      ┌──────────┐
│   Link   │······│   Link   │    │   Link   │······│   Link   │
└──────────┘      └──────────┘    └──────────┘      └──────────┘
   ct320-1           router	     acushla            salem     
192.168.110.101  192.168.110.1    129.82.47.50      129.82.46.233   

Layers and Services

Stalin says that “Quanity has a quality all its own”

• Service provided by TCP to HTTP:
  • reliable transmission of data over a logical connection
• Service provided by IP to TCP:
  • unreliable transmission of IP datagrams across an IP network
• Service provided by Ethernet to IP:
  • transmission of a frame across an Ethernet segment
• Other services:
  • DNS: translation between domain names and IP addresses
  • ARP: Translation between IP addresses and MAC addresses

Ethernet

• Computer–computer shouting-based protocol
• Each device has unique MAC (Media Access Control) address (48-bit)
  • Official style: 00-C0-4F-48-47-93
  • Linux style: 00:C0:4F:48:47:93

(Note lack of IP addresses.)

IP: Internet Protocol

• Unreliable connectionless datagram delivery service
• Responsible for routing of data through intermediate networks and computers

Protocol: 1=ICMP, 6=TCP, 17=UDP

IP Routing

┌─────────────┐                                   ┌─────────────┐
│ Application │                                   │ Application │
├─────────────┤                                   ├─────────────┤
│  Transport  │                                   │  Transport  │
├─────────────┤          ┌─────────────┐          ├─────────────┤
│   Network   │          │   Network   │          │   Network   │
├─────────────┤          ├─────────────┤          ├─────────────┤
│     Link    │··········│     Link    │··········│     Link    │
└─────────────┘          └─────────────┘          └─────────────┘

• Every device that supports the Network Layer must have a Routing Table.
• It’s a series of entries (a table) where each entry contains:
  • Destination IP address (or range or pattern)
  • IP address of a next-hop router
  • Flags
  • Network interface specification

ICMP : Internet Control Message Protocol

• Used to report problems with delivery of IP Datagrams within an IP network
• Used by ping, traceroute commands

• Echo Request (type=8, code=0)
• Echo Reply (type=0, code=0)
• Destination Unreachable (type=3, code=various)
• Time Exceeded (type=11, code=0) : Time-To-Live=0

TCP : Transmission Control Protocol

• Reliable transmission, based on unreliable IP packets.
• Keep trying until it works.
• Fragment/reassemble if needed.
• Contains no IP addresses—that’s the IP layer’s problem.

Stupid IP Address Tricks

These all work in my browser:

http://www.cs.colostate.edu
http://%77%77%77%2e%63%73%2e%63%6f%6c%6f%73%74%61%74%65%2e%65%64%75
http://129.82.45.114
http://0x81.0x52.0x2d.0x72
http://0201.0122.055.0162
http://2169646450
http://0x81522d72
http://020124426562

IP Addressing: Introduction

┌─────────────────┐       ┌───────────────┐       ┌───────────────┐
│     ct320-1     │  lan1 │    router     │       │     boise     │
│ 192.168.110.101 │·······│ 192.168.110.1 │   ····│ 129.82.46.197 │
└─────────────────┘   :   ├ ─ ─ ─ ─ ─ ─ ─ ┤   :   └───────────────┘
┌─────────────────┐   :   │    acushla    │   :   ┌───────────────┐
│     ct320-2     │   :   │ 129.82.47.50  │·······│     salem     │
│ 192.168.110.102 │····   └───────────────┘  lan2 │ 129.82.46.233 │
└─────────────────┘                               └───────────────┘

• IP address: 32-bit identifier for host, router interface
• interface: connection between host/router and physical link
  • routers typically have multiple interfaces
  • host may have multiple interfaces
  • IP addresses associated with each interface

IP Addresses

• IP (Version 4) addresses are 32 bits long
• IP addresses assigned statically or dynamically (DHCP)
• IPv6 addresses are 128 bits long
  • How many IPv6 addresses are there?
  • How many IPv6 addresses per person?

Old-Fashioned Classful Addresses

• HP owns the class A network 15.*.*.*, and DEC/Compaq’s 16.*.*.*.
  That’s 1⁄128th of the entire IP address space!
• CSU owns the class B network 129.82.*.*.
• Many home networks are private class C: 192.168.0.* or 192.168.1.*.
• My home network is the private class A network 10.*.*.*.
  I don’t need sixteen million hosts, but it’s easier to type.

IP Address Space

• Originally, 3 Classes
  • A, B, C
• Problem
  • Classes too rigid (C too small, B too big)
• Solution
  • Subnetting (e.g. within CSU)
  • Classless Interdomain Routing (CIDR)

Subnetting

• IP Address plus subnet mask (netmask)
• IP Addr: 171.64.15.82
  • Netmask: 0xFFFFFF00 (binary: 11111111 11111111 11111111 00000000)
  • First 24 bits are the Subnet ID (the neighborhood)
  • Last 8 bits are Host ID (the street address)
• Can be written as prefix/length
  • 171.64.15.0/24 or 171.64.15/24
  • I suppose that 10.11.12.13/8 is the same as 10.0.0.0/8, but don’t do that.

Subnets

┌─────────────────┐       ┌───────────────┐       ┌───────────────┐
│     ct320-1     │  lan1 │    router     │       │     boise     │
│ 192.168.110.101 │·······│ 192.168.110.1 │   ····│ 129.82.46.197 │
└─────────────────┘   :   ├ ─ ─ ─ ─ ─ ─ ─ ┤   :   └───────────────┘
┌─────────────────┐   :   │    acushla    │   :   ┌───────────────┐
│     ct320-2     │   :   │ 129.82.47.50  │·······│     salem     │
│ 192.168.110.102 │····   └───────────────┘  lan2 │ 129.82.46.233 │
└─────────────────┘                               └───────────────┘

• IP address:
  • subnet part (high order bits)
  • host part (low order bits)
• What’s a subnet?
  • device interfaces with same subnet part of IP address
  • can physically reach each other without intervening router

Subnets

┌─────────────────┐       ┌───────────────┐       ┌───────────────┐
│     ct320-1     │  lan1 │    router     │       │     boise     │
│ 192.168.110.101 │·······│ 192.168.110.1 │   ····│ 129.82.46.197 │
└─────────────────┘   :   ├ ─ ─ ─ ─ ─ ─ ─ ┤   :   └───────────────┘
┌─────────────────┐   :   │    acushla    │   :   ┌───────────────┐
│     ct320-2     │   :   │ 129.82.47.50  │·······│     salem     │
│ 192.168.110.102 │····   └───────────────┘  lan2 │ 129.82.46.233 │
└─────────────────┘                               └───────────────┘

• To determine the subnets, detach each interface from its host
  or router, creating islands of isolated networks.
  Each isolated network is a subnet.
• The left subnet is 192.168.110/24 or perhaps 192.168.110.0/25.
  The right subnet is 129.82/16 or perhaps 129.82.46/23.
• The router will not forward 192.168/16 packets to the right,
  or 129.82/16 packets to the left. Less LAN traffic! ☺

Why Have Subnets?

Why bother with routers or subnets? Just put all of your computers
on one giant LAN, and be done with it!

• Remember, Ethernet is shouting-based (broadcast).
• Only one device can talk on Ethernet at once.
• If A & B are exchanging a lot of data, so are C & D,
  then it’s best if A & B are on one LAN, and C & D are on another.

IP Addressing: CIDR

CIDR: Classless Inter Domain Routing

• byte boundaries are not significant
• subnet portion of address of arbitrary length
• address format: a.b.c.d/x, where x is # bits in subnet portion of address
  or a.b.c/x
  or a.b/x
  or a/x
• 129.82.0.0/16 or 129.82.0/16 or 129.82/16

Getting a datagram from source to destination

Consider this unusual network. In the center is a router,
with several LAN ports, connecting three LANs.
Note that 217.1.145.12 and 217.1.99.66
are also connected by a dedicated super-high-speed LAN,
because they exchange tons of data.

┌───────────┐       ┌───────────┐       ┌──────────────┐
│ 10.12.1.3 │·······│ 10.12.1.1 │   ····│ 217.1.34.252 │
└───────────┘   :   ├ ─ ─ ─ ─ ─ ┤   :   └──────────────┘
┌───────────┐   :   │ 217.1.2.1 │···:   ┌──────────────┐
│ 10.12.1.5 │···:   ├ ─ ─ ─ ─ ─ ┤   :···│ 217.1.145.12 │
└───────────┘   :   │ 22.3.4.18 │   :   └──────────────┘
┌───────────┐   :   └───────────┘   :          :
│ 10.12.1.6 │····         :         :   ┌──────────────┐
└───────────┘             :         ····│ 217.1.99.66  │
		      to Comcast        └──────────────┘

Getting a datagram from source to destination

┌───────────┐       ┌───────────┐       ┌──────────────┐
│ 10.12.1.3 │·······│ 10.12.1.1 │   ····│ 217.1.34.252 │
└───────────┘   :   ├ ─ ─ ─ ─ ─ ┤   :   └──────────────┘
┌───────────┐   :   │ 217.1.2.1 │···:   ┌──────────────┐
│ 10.12.1.5 │···:   ├ ─ ─ ─ ─ ─ ┤   :···│ 217.1.145.12 │
└───────────┘   :   │ 22.3.4.18 │   :   └──────────────┘
┌───────────┐   :   └───────────┘   :          :
│ 10.12.1.6 │····         :         :   ┌──────────────┐
└───────────┘             :         ····│ 217.1.99.66  │
		      to Comcast        └──────────────┘

How would 217.1.99.66 send data to:

• 217.1.34.252?
• 217.1.145.12?
• 10.12.1.5?
• 1.2.3.4?

How’s it supposed to know all that‽

Routing Tables

┌───────────┐       ┌───────────┐       ┌──────────────┐
│ 10.12.1.3 │·······│ 10.12.1.1 │   ····│ 217.1.34.252 │
└───────────┘   :   ├ ─ ─ ─ ─ ─ ┤   :   └──────────────┘
┌───────────┐   :   │ 217.1.2.1 │···:   ┌──────────────┐
│ 10.12.1.5 │···:   ├ ─ ─ ─ ─ ─ ┤   :···│ 217.1.145.12 │
└───────────┘   :   │ 22.3.4.18 │   :   └──────────────┘
┌───────────┐   :   └───────────┘   :          :
│ 10.12.1.6 │····         :         :   ┌──────────────┐
└───────────┘             :         ····│ 217.1.99.66  │
		      to Comcast        └──────────────┘

Each host (remember, that includes end-user computers and routers) has
a routing table. Here’s a routing table for 217.1.99.66:

Is the second entry really needed? Note the the table does not
try to get a packet all the way to 10.12.1.5; it only specifies
the next hop.

Routing Tables

┌───────────┐       ┌───────────┐       ┌──────────────┐
│ 10.12.1.3 │·······│ 10.12.1.1 │   ····│ 217.1.34.252 │
└───────────┘   :   ├ ─ ─ ─ ─ ─ ┤   :   └──────────────┘
┌───────────┐   :   │ 217.1.2.1 │···:   ┌──────────────┐
│ 10.12.1.5 │···:   ├ ─ ─ ─ ─ ─ ┤   :···│ 217.1.145.12 │
└───────────┘   :   │ 22.3.4.18 │   :   └──────────────┘
┌───────────┐   :   └───────────┘   :          :
│ 10.12.1.6 │····         :         :   ┌──────────────┐
└───────────┘             :         ····│ 217.1.99.66  │
		      to Comcast        └──────────────┘

A routing table for the router:

Routing Tables

┌───────────┐       ┌───────────┐       ┌──────────────┐
│ 10.12.1.3 │·······│ 10.12.1.1 │   ····│ 217.1.34.252 │
└───────────┘   :   ├ ─ ─ ─ ─ ─ ┤   :   └──────────────┘
┌───────────┐   :   │ 217.1.2.1 │···:   ┌──────────────┐
│ 10.12.1.5 │···:   ├ ─ ─ ─ ─ ─ ┤   :···│ 217.1.145.12 │
└───────────┘   :   │ 22.3.4.18 │   :   └──────────────┘
┌───────────┐   :   └───────────┘   :          :
│ 10.12.1.6 │····         :         :   ┌──────────────┐
└───────────┘             :         ····│ 217.1.99.66  │
		      to Comcast        └──────────────┘

A routing table for 10.12.1.6:

It only has one LAN interface. Everything’s got to go through that!
Once a (non-local) packet gets to the router, then it’s the router’s
task to figure out where it should go next.

ARP : Address Resolution Protocol

• 32-bit IP address → 48bit MAC address
• 128.97.89.153 → 00-C0-4F-48-47-93

Protocol

1. ARP request broadcast on Ethernet
2. Destination host (or anybody else) responds

ARP cache

• maintains the recent mappings from IP addresses to MAC addresses
• could eavesdrop on the ARP conversations of others

IP addresses: Allocation

• How does a host get an IP address?
• hard-coded by system admin in a file
  • Windows control-panel → network → configuration → tcp/ip → properties
  • Linux: It depends on the distro
• DHCP: Dynamic Host Configuration Protocol:, DHCP
  dynamically get address from a DHCP server (often the router)
  • “plug-and-play”