Operation Manual – IP Addressing and Performance
H3C S3610&S5510 Series Ethernet Switches
Chapter 1 IP Addressing Configuration
1-3
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.
shows how a Class B network is subnetted.
Figure 1-2
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.
1.2 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.
