In 1985, RFC
950 defined a standard procedure to support the subnetting, or division, of a
single Class A, B, or C network number into smaller pieces. Subnetting was
introduced to overcome some of the problems that parts of the Internet were
beginning to experience with the classful two-level addressing hierarchy:
Internet routing
tables were beginning to grow.
Local
administrators had to request another network number from the Internet
before a new network could be installed at their site.
Both of
these problems were attacked by adding another level of hierarchy to the IP
addressing structure. Instead of the classful two-level hierarchy, subnetting
supports a three-level hierarchy. The basic idea of subnetting is to divide the
standard classful host-number field into two parts - the subnet-number and the host number on that subnet.
Subnetting
attacked the expanding
routing table problem by ensuring that the subnet
structure of a network is never visible outside of the organization's private
network. The route from the Internet to any subnet of a given IP address is the
same, no matter which subnet the destination host is on. This is because all
subnets of a given network number use the same network-prefix but different
subnet numbers. The routers within the private organization need to
differentiate between the individual subnets, but as far as the Internet
routers are concerned, all of the subnets in the organization are collected
into a single routing table entry. This allows the local administrator to introduce
arbitrary complexity into the private network without affecting the size of the
Internet's routing tables. Subnetting
overcame the registered
number issue by assigning each organization one (or at
most a few) network number(s) from the IPv4 address space. The organization was
then free to assign a distinct subnetwork number for each of its internal
networks. This allows the organization to deploy additional subnets without
needing to obtain a new network number from the Internet.
Extended-Network-Prefix
Internet
routers use only the network-prefix of the destination address to route traffic
to a subnetted environment. Routers within the subnetted environment use the extended network- prefix to route traffic between the individual subnets. The extended-network prefix is composed of the classful network-prefix and the
subnet-number.
The
extended-network-prefix has traditionally been identified by the subnet mask.
For example, if you have a Class B address of 130.5.0.0 and you want to use the
entire third octet to represent the subnet-number, you need to specify a subnet
mask of 255.255.255.0. The bits in the subnet mask and the Internet address
have a one-to-one correspondence. The bits of the subnet mask are set to 1 if
the system examining the address should treat the corresponding bit in the IP
address as part of the extended-network-prefix. The bits in the mask are set to
0 if the system should treat the bit as part of the host-number.
The
standards describing modern routing protocols often refer to the
extended-network-prefix- length rather than the subnet mask. The prefix length
is equal to the number of contiguous one-bits in the traditional subnet mask.
This means that specifying the network address 130.5.5.25 with a subnet mask of
255.255.255.0 can also be expressed as 130.5.5.25/24. The
/<prefix-length> notation is more compact and easier to understand
than writing out the mask in its traditional dotted-decimal format. However, It is important to note that modern routing protocols still carry the subnet mask.
There are no Internet standard routing protocols that have a one-byte field in
their header that contains the number of bits in the extended-network prefix.
Rather, each routing protocol is still required to carry the complete
four-octet subnet mask.
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