Variable Length Subnet Table for IPv4
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T. Pummill, B. Manning · 1995-12
Abstract
RFC 1878 provides a reference table of all 32 IPv4 prefix lengths, listing for each the corresponding subnet mask in dotted-decimal notation, the number of addressable hosts, and the number of networks per class boundary. It serves as a practical reference for network engineers performing variable-length subnet masking (VLSM) calculations.
Why This RFC Matters
Before CIDR and VLSM became widely understood, network engineers frequently miscalculated subnet boundaries and host counts. RFC 1878's comprehensive lookup table became a classic desk reference, helping operators correctly allocate address space without wasting large blocks on small networks. Although modern tooling automates these calculations, RFC 1878 remains a concise canonical reference for understanding IPv4 subnetting and is widely cited in networking education.