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we divide it into smaller networks by extending the network prefix from 16 bits to, say 24 bits, the network uses its third
byte to subnet this class B network. The subnet mask is 255.255.255.0; each bit of the first three bytes is 1. The first two
bytes are used to identify the class B network, the third byte is used to identify the subnet within this class B network, and
the last byte is the host number.
Not all IP addresses are available in the subnetted network. Two special addresses are reserved. They are the addresses
with all zeros and all ones host number.
As shown in the table below, the subnet mask with a 25-bit long, 255.255.255.128 address contains 126 members in the
subnetted network. The network prefix length equals the bit number with 1s in that subnet mask. Use this table to count
the number of IP addresses matched.
Table 6-2. Subnet mask values.
Prefix Length Number of IPs Matched Number of Addressable IPs
/32 1 -
/31 2 -
/30 4 2
/29 8 6
/28 16 14
/27 32 30
/26 64 62
/25 128 126
/24 256 254
/23 512 510
/22 1024 1022
/21 2048 2046
/20 4096 4094
/19 8192 8190
/18 16384 16382
/17 32768 32766
/16 65536 65534
According to the scheme above, a subnet mask 255.255.255.0 will partition a network with the class C. It means that a
maximum of 254 effective nodes will exist in this sub-netted network and is considered a physical network in an
autonomous network. So it owns a network IP address such as 168.1.2.0.
With the subnet mask, a bigger network can be cut into small pieces of network. If we want to have more than two
independent networks in a worknet, a partition to the network must be performed. In this case, subnet mask must be
applied.
For different network applications, the subnet mask may be 255.255.255.240. This means it is a small network
accommodating a maximum of 15 nodes in the network.
6.3 Default Gateway
For the routed packet, if the destination is not in the routing table, all the traffic is put into the device with the designated IP
address, known as default router. Basically, it is a routing policy. The gateway setting is used for Trap Events Host only in
the switch.
To assign an IP address to the switch, you just have to check what the IP address of the network is that will be connected
to the switch. Use the same network address and append your host address to it.