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CT320 IP Addresses

Port 49757
198.51.100.135 Port 22
203.0.113.42

Format

An IPv4 (as opposed to the newer IPv6) IP address is a 32-bit quantity which represents a host on the internet. It is usually represented as four numbers, each 0–255, separated by dots. For example:

    198.51.100.106

There are 2³² (~4.2 billion) IP addresses.
This is a ridiculously large number.
We’re running out.

Grouping

It is often useful to group IP addresses.
For example, Hewlett-Packard owns all the 16.whatever IP addresses. 🐷
- Yeah, well, the DoD owns nearly five percent of all IP addresses!! 🐷🐷🐷🐷🐷
What notation shall we use, other than “whatever”?

Old-Fashioned Classful Addresses

How address ranges *used* to be allocated
Class	Pattern	Networks	Hosts/network	Addresses
A	0nnnnnnn hhhhhhhh hhhhhhhh hhhhhhhh	2⁷	2²⁴ (~16 million)	0–127.whatever
B	10nnnnnn nnnnnnnn hhhhhhhh hhhhhhhh	2¹⁴	2¹⁶ (65536)	128–191.whatever
C	110nnnnn nnnnnnnn nnnnnnnn hhhhhhhh	2²¹	2⁸ (256)	192–223.whatever

Class A: IBM (380,000 employees in 2016)
or the U.S. Postal Service (639,789 employees in 2017)
Class B: CSU (33,198 students in 2016)
Class C: Jack’s house (20 hosts in 2017)

List of assigned class A networks

Classful no good

Originally, three classes:
- A, B, C
Problem
- Classes too rigid (C too small, B too big)
Solution
- Rather than having only three possibilities:
  - 8-bit class A network
  - 16-bit class B network
  - 24-bit class C network
- Have a variable-sized network:
  - 30 bits, 23 bits, you name it

Masking

Consider a small business that has been allocated the addresses 198.51.100.0 – 198.51.100.255.
- First 24 bits are the Network ID (the neighborhood)
- Last 8 bits are Host ID (the street address)
- Netmask: FF FF FF 00₁₆ (11111111 11111111 11111111 00000000₂)
- The 1 bits represent the network
- The 0 bits represent the host
- All the 1 bits in the subnet mask are on the left
- What a stupid system!
  - Still used by some Linux networking commands

CIDR

Subnet masks are old-fashioned
- Use CIDR (Classless Inter-Domain Routing) instead.
prefix/length
- 198.51.100/24 (omit trailing zero bytes)
- 198.51.100.0/24 (also ok)
- 198.51.100.42/24 (invalid—the last 8 (32−24) bits must be zeroes)
Does not have to be on byte boundaries.
- 198.51.100.128/27 is acceptable.
- Represents addresses 198.51.100.128 through 198.51.100.159.
- How many hosts is that?

CIDR example

Consider the CS network, 129.82.44/23

10000001.01010010.00101100.00000000 = 129.82.44.0
Make the left 23 bits, the network, red, and the remaining bits, the host, blue.
10000001.01010010.00101100.00000000 = 129.82.44.0
Now, change all the blue bits from 0 to 1:
10000001.01010010.00101101.11111111 = 129.82.45.255

∴ 129.82.44/23 is the range of addresses 129.82.44.0 … 129.82.45.255.
It contains 129.82.44.whatever and 129.82.45.whatever, for 512 addresses total.

Special classes of addresses

$ host art
art.edu has address 3.233.19.193
art.edu mail is handled by 0 us-smtp-inbound-1.mimecast.com.
art.edu mail is handled by 0 us-smtp-inbound-2.mimecast.com.
$ host denver
$ host tuba
Host tuba not found: 3(NXDOMAIN)

CSU: 129.82/16
- CSU CS: 129.82.44/23
Documentation: 192.0.2/24, 198.51.100/24, 203.0.113/24
Private use: 10/8, 172.16/12, 192.168/16
- CSB 315: 192.168.110/24, a subset of 192.168/16
- CS printers: 10/8; only our subnet can access them!

I’m sorry that CSU addresses begin with 129, which is so similar to the 192 that begins one form of private networks.

`/etc/hosts`

/etc/hosts is the first stage of translating a name to an IP address:

$ grep "ub" /etc/hosts
10.1.44.62	tuba.cs.colostate.edu tuba
129.82.44.223	rubens.cs.colostate.edu rubens
129.82.45.169	rhubarb.cs.colostate.edu rhubarb
129.82.45.204	ruby.cs.colostate.edu ruby

$ grep -oP '^(\d+\.){3}' /etc/hosts | sort | uniq -c | pr -2t
      2 10.1.101.		    	 31 129.82.119.
      8 10.1.118.		    	 11 129.82.138.
    116 10.1.44.		    	246 129.82.208.
     10 10.1.45.		    	203 129.82.44.
      1 10.2.44.		    	171 129.82.45.
      1 104.154.34.		    	  5 192.168.100.
      1 127.0.0.		    	  1 255.255.255.
     74 129.82.118.

Stupid IP Address Tricks

These all work in my browser:

    http://cs.colostate.edu

    http://%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

CT320: Network and System Administration

Fall 2019

IP Addresses