Lecture 11: The IP Protocol #1

CIDR Addresses

A CIDR address has some of the characteristics of a subnetted Class A, B or C address, see earlier. They are written as "aa.bb.cc.dd/x". The new "/x" specifier indicates how many bits of the address, starting from the MSB, are to be interpreted as the "network" part, leaving the remainder to be interpreted as "host" part -- this is obviously related to the older "subnet" model.

For example, suppose an organisation needed about 1000 IP addresses for its Internet-connected hosts. A suitable CIDR allocation would be (eg)203.100.100.0/22. This means that the address part is 22 bits, and the host part is 10 bits, giving 1024 host IDs. As usual, we write all zeros in the host part of the address when referring to the "network" itself.

An additional characteristic of CIDR is that IP address blocks are now allocated on a geographic basis, or more correctly, on the basis of domain names. So, for example, virtually all IP addresses recently allocated in Australia now have 203 as their first byte. This is done to simplify routing, see later

The tutorial for this lecture is Tutorial #11.
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