Subnetting
Connected: An Internet Encyclopedia
Subnetting
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Section 3 - The IP Protocol
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Subnetting
Subnetting
Engineers gradually realized that the original class-based addressing
design was too inflexible for many applications.
In the mid-1980s, subnetting was introduced as an
extension to the original scheme.
Subnetting, documented in
RFC 950,
originally referred to the subdivision of a class-based
network into subnetworks, but now refers more generally to the subdivision
of a CIDR block into smaller CIDR blocks. Subnetting allows single
routing entries to refer either to the larger block or to its individual
constituents. This permits a single, general routing entry to be used
through most of the Internet, more specific routes only being required
for routers in the subnetted block.
A subnet mask is a 32-bit number that
determines how an IP address is split into network
and host portions, on a bitwise basis. For example, 255.255.0.0 is
a standard class B subnet mask, since the first two bytes are all ones
(network), and the last two bytes are all zeros (host).
In a subnetted network, the network portion is extended. For example,
a subnet mask of 255.255.255.0 would subnet a class B address space
using its third byte. Using this scheme, the first two bytes of an
IP address would identify the class B network, the next byte would
identify the subnet within that network, and the final byte would
select an individual host. Since subnet masks are used on a bit-by-bit
basis, masks like 255.255.240.0 (4 bits of subnet; 12 bits of host)
are perfectly normal.
In a traditional subnetted network, several restrictions apply, which
have been lifted by CIDR. However, if older, non-CIDR routing protocols
(such as RIP version 1) are in use, these restrictions must still
be observed.
- Identical subnet masks. Since non-CIDR routing updates
do not include subnet masks, a router must assume that the subnet mask it
has been configured with is valid for all subnets.
Therefore, a single mask must be used for all subnets with a
network. Different masks can be used for different networks.
Based on this assumption, a router can exchange subnet routes
with other routers within the network. Since the subnet masks
are identical across the network, the routers will interpret
these routes in the same manner. However, routers not attached
to the subnetted network can't interpret these subnet routes,
since they lack the subnet mask.
Therefore, subnet routes are not relayed to routers on other
networks. This leads to our second restriction.
- Contiguous subnets. A subnetted network
can't be split into isolated portions. All the subnets must
be contiguous, since routing information can't be passed to
non-members. Within a network, all subnets must be able to
reach all other subnets without passing traffic through
other networks.
Variable Length Subnet Masks (VLSM)
VLSM, conceptually a stepping stone from subnetting to CIDR,
lifted the restrictions of subnetting by relaying subnet
information through routing protocols. This idea leads us
directly to CIDR.
Next: CIDR
Connected: An Internet Encyclopedia
Subnetting
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