Subnet Mask - Black Box 12-Port 1000BASE-TX L2 Managed PoE Switch User Manual

12-Port 1000BASE-TX L2 Managed PoE Switch with 2 SFP Dual Media Ports
3.5.2 S M
UBNET ASK
Subnet mask is the sub-division of a class-based network or a CIDR block. The subnet is used to determine how
to split an IP address to the network prefix and the host address. It's designed to use an IP address more
efficiently to manage an IP network.
For a class B network, 128.1.2.3, the default subnet mask may be 255.255.0.0. The first two bytes are all 1s. This
means more than 60 thousands of nodes in flat IP addresses will be on the same network. It's too large to
manage practically. If we divide it into smaller networks by extending the network prefix from 16 bits to, say
24bits, 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.
Prefix Length
/32
/31
/30
/29
/28
/27
/26
/25
/24
/23
/22
/21
/20
/19
/18
/17
/16
According to the table above, a subnet mask 255.255.255.0 will partition a network with the class C. This means
that a maximum of 254 effective nodes exist in this subnetted network and it's considered a physical network in
an autonomous network. A sample network IP address is 168.1.2.0.
With the subnet mask, for more than two independent networks in a worknet, the network can be partitioned
into smaller networks. A subnet mask must be applied.
For different network applications, a sample subnet mask is 255.255.255.240. This is for a small network with a
maximum of 15 nodes.
26
Table 3-3. Subnet mask values.
Number of IPs Matched
1
2
4
8
16
32
64
128
256
512
1024
2048
4096
8192
16384
32768
65536
Number of Addressable IPs
-
-
2
6
14
30
62
126
254
510
1022
2046
4094
8190
16382
32766
65534