VLSM

The Problem VLSM Solves

Classful IP addressing allocated fixed-size blocks (Class A, B, C) regardless of actual host requirements, wasting enormous amounts of address space. Variable Length Subnet Masking (VLSM) allows network engineers to carve an IP block into subnets of different sizes using different prefix lengths — a /30 for a point-to-point WAN link (2 usable hosts) alongside a /24 for a large office LAN (254 usable hosts) — all within the same parent CIDRClassless Inter-Domain Routing. A method of allocating IP addresses using variable-length subnet masking (e.g., 10.0.0.0/8) instead of fixed classful boundaries, enabling more efficient use of the IPv4 address space. block.

How VLSM Works

Starting with a parent prefix, you divide it hierarchically: allocate the largest required subnet first, then continue subdividing remaining space for smaller requirements. For example, given 192.168.1.0/24, you might carve out 192.168.1.0/25 (126 hosts), 192.168.1.128/26 (62 hosts), 192.168.1.192/27 (30 hosts), and 192.168.1.224/30 (2 hosts) for a WAN link. VLSM requires routing protocols that carry Subnet MaskA 32-bit number (e.g., 255.255.255.0) that divides an IP address into network and host portions. It determines which part of the address identifies the network and which part identifies individual devices. information in their updates — classless protocols like OSPF, EIGRP, and BGP support this, while older RIPv1 does not.

Design Best Practice

Always allocate from largest to smallest to minimize fragmentation of address space. Document every assignment in an IP address management (IPAM) system. VLSM is the foundation of efficient Private IP AddressAn IP address from reserved ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) used within local networks. Private addresses are not routable on the public internet and require NAT for external communication. space management in enterprise networks. Subnet Calculator