A subnet mask is a 32-bit address that segregates an IP address into network bits that identify the network and host bits that identify the host device operating on that network. (Reference: 03-Nov-2022)
Understanding Subnet Masks in Detail
Subnet masks are crucial for network communication, allowing devices to determine whether traffic should be sent within the local network or to a router for delivery to a different network. Here's a more detailed explanation:
How Subnet Masks Work
A subnet mask essentially acts as a filter, applied to an IP address to distinguish the network portion from the host portion. It's a string of ones followed by a string of zeros.
- Network Bits (1s): The '1' bits in the subnet mask identify the network portion of the IP address. All devices on the same network share the same network address.
- Host Bits (0s): The '0' bits in the subnet mask identify the host portion of the IP address. This part uniquely identifies a specific device within that network.
Examples of Subnet Masks
Common subnet masks and their corresponding CIDR notation are shown below:
| Subnet Mask | CIDR Notation | Number of Hosts |
|---|---|---|
| 255.255.255.0 | /24 | 254 |
| 255.255.0.0 | /16 | 65,534 |
| 255.0.0.0 | /8 | 16,777,214 |
| 255.255.255.128 | /25 | 126 |
Practical Insights
- Default Subnet Masks: Different IP address classes (A, B, C) have default subnet masks, but these can be customized to create subnets.
- Subnetting: The process of dividing a network into smaller subnetworks by borrowing bits from the host portion of the IP address and using them for the network portion. This is done to improve network efficiency, security, and organization.
- CIDR Notation: Shorthand for representing the subnet mask. For instance, /24 means the first 24 bits of the subnet mask are 1s (255.255.255.0).
Using Subnet Masks for Network Determination
A device uses its IP address and subnet mask to determine if another device is on the same network. It does this by performing a bitwise AND operation between its own IP address and subnet mask, and then performing the same operation on the destination IP address and the same subnet mask. If the resulting network addresses are the same, the devices are on the same network and can communicate directly. If not, traffic is routed to the default gateway.
