viral_thakkar
04-18-2008, 03:57 AM
Comments on Classless Addressing
The terms classless and classful can be applied to three popular topics that are all related to IP.
This chapter explains classful and classless IP addressing, which are relatively simple concepts. Two
other chapters explain the other uses of the terms classless and classful: Chapter 7, “IP Forwarding
(Routing),” describes classless/classful routing, and Chapter 8, “RIP Version 2,” covers classless/
classful routing protocols.
Classless IP addressing, simply put, means that class A, B, and C rules are ignored. Each address
is viewed as a two-part address, formally called the prefix and the host parts of the address. The
prefix simply states how many of the beginning bits of an IP address identify or define the group.
It is the same idea as using the combined network and subnet parts of an address to identify a
subnet. All the hosts with identical prefixes are in effect in the same group, which can be called a
subnet or a prefix.
Just as a classful subnet must be listed with the subnet mask to know exactly which addresses
are in the subnet, a prefix must be listed with its prefix length. The prefix itself is a dotteddecimal
number. It is typically followed by a / symbol, after which the prefix length is listed.
The prefix length is a decimal number that denotes the length (in bits) of the prefix. For example,
172.31.13.0/24 means a prefix of 172.31.13.0 and a prefix length of 24 bits. Also, the prefix can
be implied by a subnet mask, with the number of 1s in the binary version of the mask implying
the prefix length.
Classless and classful addressing are mainly just two ways to think about IP address formats. For
the exam, make sure to understand both perspectives and the terminology used by each.
The terms classless and classful can be applied to three popular topics that are all related to IP.
This chapter explains classful and classless IP addressing, which are relatively simple concepts. Two
other chapters explain the other uses of the terms classless and classful: Chapter 7, “IP Forwarding
(Routing),” describes classless/classful routing, and Chapter 8, “RIP Version 2,” covers classless/
classful routing protocols.
Classless IP addressing, simply put, means that class A, B, and C rules are ignored. Each address
is viewed as a two-part address, formally called the prefix and the host parts of the address. The
prefix simply states how many of the beginning bits of an IP address identify or define the group.
It is the same idea as using the combined network and subnet parts of an address to identify a
subnet. All the hosts with identical prefixes are in effect in the same group, which can be called a
subnet or a prefix.
Just as a classful subnet must be listed with the subnet mask to know exactly which addresses
are in the subnet, a prefix must be listed with its prefix length. The prefix itself is a dotteddecimal
number. It is typically followed by a / symbol, after which the prefix length is listed.
The prefix length is a decimal number that denotes the length (in bits) of the prefix. For example,
172.31.13.0/24 means a prefix of 172.31.13.0 and a prefix length of 24 bits. Also, the prefix can
be implied by a subnet mask, with the number of 1s in the binary version of the mask implying
the prefix length.
Classless and classful addressing are mainly just two ways to think about IP address formats. For
the exam, make sure to understand both perspectives and the terminology used by each.