Guide
Fitting subnets of different sizes into one address block
Splitting a network into equal parts wastes addresses when departments are different sizes. A sales floor with 100 devices and a router link with 2 should not both get a /25. Variable-length subnet masking (VLSM) gives each subnet only the size it needs. The page performs the allocation for a parent block and a list of host requirements, and shows the result as a table.
The method
First, sort the requirements from largest to smallest. For each, find the smallest block that can hold the hosts plus two reserved addresses, the network and the broadcast. Block sizes are powers of two, so 100 hosts need 102 addresses and round up to 128, a /25; 50 hosts need 52 and round up to 64, a /26. Then place the blocks one after another, each starting on a boundary that is a multiple of its own size. Largest-first order is what makes the boundaries line up without gaps; a small block put first would push the larger ones to later boundaries.
Point-to-point links
A link between two routers needs only two addresses. Normally a /30 is used (four addresses, two usable), but RFC 3021 allows a /31 on point-to-point links, using both addresses and having no broadcast. The page allocates a /31 when a requirement says two hosts, and shows the broadcast column as a dash. If your equipment does not support /31 links, plan those links by hand with a /30 and leave room for them in the parent block.
Using the page
Type the parent network, such as 192.168.1.0/24, and add a row for each subnet with a name and the number of hosts it must serve. Press Sample for a ready-made office plan. The parent must be an IPv4 network. The allocation table keeps your order and lists name, hosts needed, the subnet in CIDR notation, mask, usable host range, broadcast address and usable count, and the summary line shows how many addresses are left unused. Copy as CSV puts the table into a spreadsheet or a documentation page.
A worked example
The sample parent is 192.168.1.0/24 with five requirements. Sales needs 100 hosts and gets 192.168.1.0/25 (126 usable). Office needs 50 and gets 192.168.1.128/26 (62 usable). Guests needs 25 and gets 192.168.1.192/27 (30 usable). Lab needs 10 and gets 192.168.1.224/28 (14 usable). The router link needs 2 and gets 192.168.1.240/31. The blocks use 128 + 64 + 32 + 16 + 2 = 242 addresses, leaving 14 unused in the /24, which is the tail from 192.168.1.242 to 192.168.1.255.
Limits
The plan covers one parent block and assumes you want aligned, power-of-two subnets. It does not model growth: if a department may double, enter the future size. It does not know about your VLAN numbers, routing or DHCP scopes, and the layout is one valid solution, not the only one. For single subnet arithmetic use the Subnet Calculator.
Related tools
- Subnet Calculator works out one network in detail.
- Route Summarization Calculator combines allocated subnets into one advertisement.
- Wildcard Mask Calculator writes the access-list masks for each subnet.
- CIDR to IP Range lists first and last addresses of each block.
Checked against the current version of the VLSM Calculator on .