Subnet mask: A subnet mask is a mechanism used to split a network into subnetworks; it can be used to reduce the traffic on each subnetwork by confining traffic to only the subnetwork(s) for which it is intended, thereby eliminating issues of associated congestion on other subnetwork(s) and reducing congestion in the network as a whole. Each subnet functions as though it were independent, keeping traffic local and forwarding traffic to another subnetwork only if the address of the data is external to the subnetwork.
Subnetting an IP network allows for the flow of network traffic to be segregated based on a network configuration. It essentially organizes the hosts into logical groups, and provides for improving network security and performance. The most common reason for subnetting IP networks is to control network traffic. Traditionally, in an Ethernet network, it is very common for all nodes on a segment to see all the packets transmitted by all the other nodes on that segment, which introduces collisions, and the resulting retransmissions under heavy traffic loads. For additional information on subnetting, see RFC 1817, and RFC 1812.
Class Address Ranges
Class A - 126.96.36.199 to 188.8.131.52
Class B - 184.108.40.206 to 220.127.116.11
Class C - 18.104.22.168 to 22.214.171.124
Class D* - 126.96.36.199 to 188.8.131.52
Class E* - 240.0.0.0 to 255.255.255.255
Class A, Class B, and Class C are the three classes of addresses used on IP networks in common practice. Class D addresses are reserved for multicast. Class E addresses are simply reserved, meaning they should not be used on IP networks (used on a limited basis by some research organizations for experimental purposes).
Reserved Address Ranges
Address ranges below are reserved by IANA for private intranets, and not routable to the Internet. For additional information, see RFC 1918.
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
Other reserved addresses:
127.0.0.0 is reserved for loopback and IPC on the localhost.
184.108.40.206 - 220.127.116.11 is reserved for multicast addresses.
255.255.255.255 is the limited broadcast address (limited to all other nodes on the LAN)
Subnet Calculator Explanation(Example)
This calculator will calculate the subnet mask to use, given a TCP/IP network address and the number of subnets or nodes per subnet required.
To create the subnet mask, first remember that the purpose of the subnet mask is to separate the (32 bit) ip address into the network prefix and the host number. If a bit in the subnet mask is 1, the corresponding bit in the IP address is part of the network address; if the bit in the subnet mask is 0, the corresponding bit in the IP address is part of the host address.
First depict the ip address in binary. Take 18.104.22.168 and convert to binary:
ip address: 00111101.11110110.00010011.00010010
First we determine what class of address it is:
If the first bit is 0 it is a Class A address If the first two bits are 10 it is a Class B address If the first three bits are 110 it is a Class C address If the first four bits are 1110 it is a Class D multicast address If the first four bits are 1111 it is a Class E experimental address
Your example is a Class A address. The default subnet mask for a Class A address is:
subnet mask: 11111111.00000000.00000000.00000000
The formula for figuring out the number of 'host' bits in a subnet mask is
2^n=(number of nodes ) (2^n means '2' to the power of 'n')
Since you know the number of nodes, you need to find 'n'.
Because you want 64 node(s), you want to leave 6 - '0' bits in the subnet mask since 64 = 2 ^ 6.
This will give you the following subnet mask:
subnet mask: 11111111.11111111.11111111.11000000
Which is referred to as /26 or in dotted decimal notation as 255.255.255.192