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:

    198.51.100.135

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.

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

CT320: Network and System Administration

Fall 2018

IP Addresses