Tuesday, 12 December 2006

Some Notes on IP Addressing ...

I have cobbled together these notes that explain some of the finer points regarding IP Addressing.

IP Addresses

An IP Address is split into a Network Portion and a Host Portion

Host Numbers cannot be Zero or 255. All Zeros in the Host Area refers to the network itself: All Host bits to 255 is the Broadcast Address. For Network 203.176 the Broadcast Address is

A Class A Network allows 16,777,216 Hosts
A Class B Network allows 16,384 Hosts
A Class C Network allows 254 Hosts

Looking at the first octet of the 32 bit address you can determine what class of address it is: A value of 126 or less means that you are looking at a Class A address, 127 is the loopback address, 128 through 191 is a Class B address and 192 through 223 is a Class C address. Numbers above 223 are reserved.

The chart below shows this in an easily digestible format:

1 > 126 Class A
128 > 191 Class B
192 > 223 Class C

RFC 1918 gives the address range 192.168.XXX.XXX as available to anybody to use for private LAN networking. In addition the 10.XXX.XXX.XXX network and 172.16.XXX.XXX networks are also available for private use. These addresses will not work on the internet as they are non routable.

Subnet Masks: Class A Class B Class C

Classless Internetwork Domain Routing (CIDR)

CIDR Networks are described as Slash X Networks. X is the number of bits in the IP Address range that ICANN controls, you get what's left. For example a Class C is known as a Slash 24 Network since ICANN has the left most 24 Bits and you have the right most 8 Bits. See examples below:

ICANN Subnet Mask
Slash 8
Slash 16
Slash 24
Slash 28

"Do not trust all men, but trust men of worth; the former course is silly, the latter a mark of prudence." - Democritus

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