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:

```    198.51.100.107
```
• 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”?

How address ranges used to be allocated
A0nnnnnnn hhhhhhhh hhhhhhhh hhhhhhhh27224 (~16 million)0–127.whatever
B10nnnnnn nnnnnnnn hhhhhhhh hhhhhhhh214216 (65536)128–191.whatever
C110nnnnn nnnnnnnn nnnnnnnn hhhhhhhh22128 (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)

# 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

• 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

• 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.

```\$ host art
\$ host denver
\$ host tuba
```
• 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.

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
```

User: Guest

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Modified: 2018-10-18T11:19

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