Subnetting And Masking - H3C S9500 Series Operation Manual

Operation Manual – IP Addressing
H3C S9500 Series Routing Switches

1.1.3 Subnetting and Masking

Subnetting was developed to address the risk of IP address exhaustion resulting from
fast expansion of the Internet. The idea is to break a network down into smaller
networks called subnets by using some bits of the host-id to create a subnet-id. To
identify the boundary between the host-id and the combination of net-id and subnet-id,
masking is used. (When subnetting is not adopted, a mask identifies the boundary
between the host-id and the host-id.)
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 1-2
Figure 1-2 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
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
you have only 7 bits for the host-id and thus have only 126 (2
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.
shows how a Class B network is subnetted.
9
) subnets by using the first 9 bits of the host-id for the subnet,
Chapter 1 IP Addressing Configuration
1-3
16
– 2. Of the two deducted
7
– 2) hosts in each subnet.