Also known as: network mask
Quick Definition
A 32-bit value that divides an IP address into the network and host portions.
A subnet mask is a 32-bit number written in dotted-decimal notation (e.g., 255.255.255.0) or as a prefix length (e.g., /24). It identifies which bits of an IP address represent the network portion and which represent the host portion. Bits set to 1 in the mask indicate the network portion; bits set to 0 indicate the host portion. All devices in the same subnet share the same network bits.
The address 192.168.1.10/24 has subnet mask 255.255.255.0. The network address is 192.168.1.0 and valid host addresses are 192.168.1.1–192.168.1.254. The broadcast address is 192.168.1.255.
Subnet masks in routing protocol configurations use wildcard masks (the inverse of the subnet mask), not the subnet mask itself. A /24 subnet mask uses wildcard mask 0.0.0.255 in ACLs and OSPF network statements.
A method of IP address allocation that uses prefix notation instead of fixed class boundaries.
Using different subnet mask lengths within the same major network to minimise IP waste.
The inverse of a subnet mask, used in ACLs and OSPF network statements.
The IP address of the router a host sends packets to when the destination is on a different subnet.
A subnet mask is a 32-bit number written in dotted-decimal notation (e.g., 255.255.255.0) or as a prefix length (e.g., /24). It identifies which bits of an IP address represent the network portion and which represent the host portion. Bits set to 1 in the mask indicate the network portion; bits set to 0 indicate the host portion. All devices in the same subnet share the same network bits.
Subnet masks in routing protocol configurations use wildcard masks (the inverse of the subnet mask), not the subnet mask itself. A /24 subnet mask uses wildcard mask 0.0.0.255 in ACLs and OSPF network statements.
The address 192.168.1.10/24 has subnet mask 255.255.255.0. The network address is 192.168.1.0 and valid host addresses are 192.168.1.1–192.168.1.254. The broadcast address is 192.168.1.255.
Subnet Mask falls under the IP Addressing domain of the 200-301 exam. Understanding it in context with related terms like cidr and vlsm is essential for answering scenario-based questions correctly.