3Com Switch 8800 Advanced Software V5 Configuration Guide

Configuring IP Addresses 207
Each subnet mask comprises 32 bits related to the corresponding bits in an IP
address. In a subnet mask, the part containing consecutive ones identifies the
combination of net-id and subnet-id whereas the part containing consecutive
zeros identifies the host-id.
Subnetting is valid with a single network. All these subnetworks 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.
Figure 61 shows how a Class B network is subnetted.
Figure 61 Subnet a Class B network
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.
IP Unnumbered Logically, to enable IP on an interface, you must assign this interface a unique IP
address. Yet, you can borrow an IP address already configured on one of other
interfaces on your device instead. This is called IP unnumbered and the interface
borrowing the IP address is called IP unnumbered interface.
You may need to use IP unnumbered to save IP addresses either when available IP
addresses are inadequate or when an interface is brought up but for occasional
use.
Configuring IP
Addresses
Besides directly assigning an IP address to an interface, you may configure the
interface to obtain one through DHCP; however, these two methods are mutually
exclusive. If you change the way an interface obtains an IP address, from manual
assignment to DHCP for example, the IP address obtained through DHCP will
overwrite the previous one manually assigned.
1
Net-id Host-id
0
Class B address
07 152331
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