What is the Purpose of the Subnet Mask?

Subnet mask is one of the numbers used with IP addressing. The role of subnet mask is defining network and host portions of an IP address. IP Addressing and IP Subnetting are important lessons for CCNA, CCNP ENCOR and CCIE level exams.

An IP address consist of two portions. These are network portion and host portion. Network part is unique in a network while host part changes to identify any specific host in a network. Subnet mask shows these parts.

In other words, with subnet mask we determine whether a destination is local or remote. And according to the subnet mask, if it is in another network, we send the traffic to the default gateway.

A subnet mask consists of a continuous 1s followed by continuous 0s in binary format but it is showed in dotted decimal format mostly.

For example, 255.255.255.0 is a subnet mask. It also shows in binary format like 11111111.11111111.11111111.0.

To shows subnet mask, generally CIDR notation is used.

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What is CIDR?

CIDR (Classless Inter-Domain Routing) is a method of representing subnet masks using a prefix length. Let’s give an example to understand what is CIDR better. Here is a CIDR notation example:

For example, if we have 192.168.1.0/24, our network portion is 192.168.1 and our host portion is .0. In other words, our network part is the first 24 bits as CIDR value shows. The remining 8 bits are host bits. So, by changing these 8 host bits we can have 256 different ip addresses. 254 of these ip addresses are usable ip addresses.

Why CIDR is important? Basically CIDR, removes classful ip addressing (A class, B class, C class) limitations. It allows efficient IP address usage. All routing protocols uses this modern notation.

Subnet Mask Questions & Interview Tips

1) What is the purpose of a subnet mask?

A subnet mask identifies which part of an IP address is the network portion and which part is the host portion.

2) How does a subnet mask affect routing decisions?

A subnet mask helps a host determine whether the destination IP address is on the local network or should be forwarded to the default gateway. In other words, Subnet masks do not route traffic, they help devices decide where to send it.

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