Network Mask: What is it and How is it Used in Routing?
In computer networking, a network mask (also known as a subnet mask) is a 32-bit value used to divide an IP address into network and host portions. It helps determine which part of the IP address represents the network identifier and which part represents the host identifier. Network masks are an essential component of routing, which enables data to be transmitted between different networks. In this blog post, we will explain what network masks are, how they are used in routing, how they are calculated, and provide examples of different masks.
What is a Network Mask?
A network mask is a binary pattern that identifies which bits of an IP address represent the network portion and which bits represent the host portion. For example, if an IP address is 192.168.1.100 and the network mask is 255.255.255.0, the first three octets (192.168.1) represent the network portion, and the last octet (100) represents the host portion. This means that all devices with IP addresses in the range 192.168.1.1 to 192.168.1.254 belong to the same network.
How is a Network Mask Used in Routing?
When a device needs to send data to another device on a different network, it first checks its routing table to determine the next hop (i.e., the next router or gateway) that can forward the data. The routing table contains a list of network addresses, associated with a corresponding network mask, and the next hop to reach that network. The network mask helps the device determine which bits of the destination IP address represent the network portion and match it with the entries in the routing table.
For example, suppose a device wants to send data to the IP address 10.20.30.40. It checks its routing table and finds the following entries:
10.0.0.0 255.0.0.0 Router A
10.20.0.0 255.255.0.0 Router B
10.20.30.0 255.255.255.0 Router C
The device compares the destination IP address with each entry in the routing table, using the network mask to determine the network portion of the address. If the first 8 bits of the destination IP address match the first 8 bits of the network address 10.0.0.0, the device sends the data to Router A. If the first 16 bits match the network address 10.20.0.0, the data is sent to Router B. If the first 24 bits match the network address 10.20.30.0, the data is sent to Router C. The routing table entries are processed in order, from the most specific to the least specific, until a match is found.
How is a Network Mask Calculated?
A network mask is calculated by setting all the bits that represent the network portion to 1 and all the bits that represent the host portion to 0. For example, a network mask of 255.255.255.0 has the binary representation 11111111.11111111.11111111.00000000, indicating that the first 24 bits represent the network portion and the last 8 bits represent the host portion.
Different Masks and Route Priorities
The choice of network mask affects the number of IP addresses available for hosts on the network and the number of networks that can be created. The following table shows some common network masks and the number of hosts and networks they support:
Network Mask Number of Hosts Number of Networks
255.255.255.128 126 2
255.255.255.192 62 4
255.255.255.224 30 8
255.255.255.240 14 16
255.255.255.248 6 32
255.255.255.252 2 64
255.255.255.254 0 128
255.255.255.255 0 1 (broadcast only)
As we can see from the table, a smaller network mask provides more networks but fewer hosts per network, while a larger network mask provides fewer networks but more hosts per network. It is essential to choose the appropriate network mask based on the number of hosts and networks required on the network.
When multiple routing table entries match a destination IP address, the device selects the entry with the longest (i.e., most specific) network mask. For example, if the routing table contains the following entries:
10.20.30.0 255.255.255.0 Router A
10.20.0.0 255.255.0.0 Router B
If the device wants to send data to the IP address 10.20.30.40, both entries match the destination IP address, but the entry with the network mask of 255.255.255.0 (i.e., Router A) is selected because it is more specific than the entry with the network mask of 255.255.0.0 (i.e., Router B).
Conclusion
Network masks are a crucial component of routing in computer networking. They help divide an IP address into network and host portions, enabling devices to transmit data between different networks. Network masks are calculated by setting all the bits that represent the network portion to 1 and all the bits that represent the host portion to 0. The choice of network mask affects the number of IP addresses available for hosts on the network and the number of networks that can be created. When multiple routing table entries match a destination IP address, the device selects the entry with the longest network mask. By understanding network masks and how they are used in routing, network administrators can effectively manage their networks and ensure efficient data transmission.
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