Configuring IP Addresses 123
Figure 36 Subnet a Class B network
While allowing you to create multiple logical networks within a single Class A, B,
or C network, subnetting is transparent to the rest of the Internet. All these
networks still appear as one. As subnetting adds an additional level, subnet-id, to
the two-level hierarchy with IP addressing, IP routing now involves three steps:
delivery to the site, delivery to the subnet, and delivery to the host.
In the absence of subnetting, some special addresses such as the addresses with
the net-id of all zeros and the addresses with the host-id of all ones, are not
assignable to hosts. The same is true of subnetting. When designing your
network, you should note that subnetting is somewhat a tradeoff between
subnets and accommodated hosts. For example, a Class B network can
accommodate 65,534 (2
16
- 2. Of the two deducted Class B addresses, one with
an all-one host-id is the broadcast address and the other with an all-zero host-id is
the network address) hosts before being subnetted. After you break it down into
512 (2
9
) subnets by using the first 9 bits of the host-id for the subnet, you have
only 7 bits for the host-id and thus have only 126 (2
7
- 2) hosts in each subnet. The
maximum number of hosts is thus 64,512 (512 × 126), 1022 less after the
network is subnetted.
Class A, B, and C networks, before being subnetted, use these default masks (also
called natural masks): 255.0.0.0, 255.255.0.0, and 255.255.255.0 respectively.
Configuring IP
Addresses
Besides directly assigning an IP address to an interface, you may configure the
interface to obtain one through BOOTP or DHCP as alternatives. If you change the
way an interface obtains an IP address, from manual assignment to BOOTP for
example, the IP address obtained from BOOTP will overwrite the old one manually
assigned.
n
This chapter only covers how to assign an IP address manually. For other
approaches, refer to “DHCP Overview” on page 791.
This section includes:
■ “Assigning an IP Address to an Interface” on page 123
■ “IP Addressing Configuration Example” on page 124
Assigning an IP Address
to an Interface
You may assign an interface multiple IP addresses, one primary and multiple
secondaries, to connect multiple logical subnets on the same physical subnet.
Follow these steps to assign an IP address to an interface:
1
Net-id Host-id
0
Class B address
0 7 15 23 31
Mask
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Net-id Host-id
Subnetting
Mask
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0
Subnet-id
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