Subnet Cheat Sheet

A subnet cheat sheet is a quick-reference guide that maps CIDR prefix lengths to subnet masks, usable host counts, wildcard masks, and block sizes. It helps network engineers, students, and IT professionals quickly look up subnetting values instead of calculating them from scratch every time.

A subnet mask is a 32-bit number that divides an IP address into a network portion and a host portion. The mask uses consecutive 1-bits for the network part and 0-bits for the host part. For example, 255.255.255.0 (/24) means the first 24 bits identify the network and the last 8 bits identify hosts, giving you 254 usable addresses per subnet.

Use the formula: usable hosts = 2^(host bits) − 2. You subtract 2 because the first address is the network address and the last is the broadcast address. For example, a /26 subnet has 6 host bits: 2^6 − 2 = 62 usable hosts. The exceptions are /31 (2 usable, RFC 3021 point-to-point) and /32 (single host route).

CIDR (Classless Inter-Domain Routing) notation represents an IP address followed by a slash and the number of network bits, for example, 192.168.1.0/24. The number after the slash tells you how many leading bits of the address are the network prefix. CIDR replaced the old classful addressing system (Class A/B/C) and allows more flexible subnet sizing.

A wildcard mask is the exact inverse of a subnet mask: wildcard = 255.255.255.255 − subnet mask. Where a subnet mask uses 1s to mark network bits and 0s for host bits, a wildcard mask uses 0s for 'must match' and 1s for 'don't care'. Wildcard masks are used in Cisco ACLs and OSPF configurations. For example, subnet mask 255.255.255.0 has wildcard mask 0.0.0.255.

RFC 1918 defines three private IP ranges that are not routable on the public internet: Class A: 10.0.0.0/8 (10.0.0.0, 10.255.255.255, ~16.7 million hosts), Class B: 172.16.0.0/12 (172.16.0.0, 172.31.255.255, ~1 million hosts), and Class C: 192.168.0.0/16 (192.168.0.0, 192.168.255.255, ~65,000 hosts). These are used for internal networks behind NAT.

VLSM (Variable Length Subnet Masking) lets you use different subnet mask sizes within the same network. Instead of giving every subnet the same size, you allocate larger subnets (e.g., /25) for segments with many hosts and smaller subnets (e.g., /30) for point-to-point links. This drastically reduces IP address waste compared to fixed-length subnetting.

Use the magic number method: (1) Find the 'interesting octet', the subnet mask octet that isn't 255 or 0. (2) Calculate the block size: 256 − interesting octet value. (3) Find the largest multiple of the block size that is ≤ your IP's octet value. That gives you the network address. For example, 192.168.10.50/26: mask octet = 192, block = 64, 50 falls in the 0-block → network is 192.168.10.0.