240 Chapter 5: Network Protocols
Figure 5-19
Address Formats, When Subnetting Is Used
The number of hosts per network or subnet is defined by the number of host bits; 2
hostbits
minus
two special reserved cases, is the number of assignable IP addresses in a network or subnet.
Similarly, the number of subnets of a network, assuming that the same mask is used on all
subnets, is defined by the number of subnet bits; 2
subnetbits
is the number of usable IP subnets
of that network. Two special cases, the "zero subnet" and "broadcast subnet," were reserved in
years past but are now usable.
Some definitions help summarize the concepts behind the address formats:
·
Two unique IP addresses in the same network have identical values in the network part of
their address and have different values in their host parts.
·
Two unique IP addresses in the same subnet have identical values in the network part of
their address, identical values in the subnet part of their address, and different values in
their host parts.
·
Two unique IP addresses in different subnets of the same Class A, B, or C network have
identical values in the network part of their address and have different values in the subnet
part of their address.
·
Without subnetting, the network number, the network broadcast address, and all
assignable IP addresses in the network have the same value in the network part of their
addresses.
·
With subnetting, the subnet number, the subnet broadcast address, and all assignable IP
addresses in the subnet have the same value in the network and subnet parts of their
addresses.
·
Most people treat the combined network and subnet parts of addresses as one part of the
address and call it the subnet part of the address, or simply the subnet.
IP Grouping Concepts and Subnetting
Cisco requires that CCNAs exhibit a thorough understanding of IP subnetting. Almost every
organization with a network uses IP, and almost every one of these organizations uses
subnetting. Subnetting is simply the process of treating subdivisions of a single Class A, B, or
Network
Host
Class A
8
x
Subnet
24 x
Network
Host
Class B
16
Subnet
16 x
x
Host
Network
Class C
x
Subnet
8 x
24
ch05.fm Page 240 Monday, March 20, 2000 5:06 PM