03 / IN THE FIELD · 5 MIN READ
Split a Network Deliberately.
Splitting borrows host bits for a longer prefix. The child blocks must remain aligned and fit exactly inside their parent.
Four Children from One /24
Start with the documentation example 192.0.2.0/24 and select four equal blocks. Four is 2², so the new prefix is /24 + 2 = /26. Each child has 64 addresses:
| Child | Inclusive Start | Inclusive End |
|---|---|---|
192.0.2.0/26 | 192.0.2.0 | 192.0.2.63 |
192.0.2.64/26 | 192.0.2.64 | 192.0.2.127 |
192.0.2.128/26 | 192.0.2.128 | 192.0.2.191 |
192.0.2.192/26 | 192.0.2.192 | 192.0.2.255 |
The ranges are adjacent, have no overlaps, and cover the parent exactly. Their combined total is still 256 addresses. But four conventional /26 host pools offer 4 × 62 = 248 endpoints, rather than the 254 conventional endpoints of the unsplit /24.
A Compact IPv4 Reference
| Prefix | Subnet Mask | Total | Conventional Endpoints |
|---|---|---|---|
| /24 | 255.255.255.0 | 256 | 254 |
| /25 | 255.255.255.128 | 128 | 126 |
| /26 | 255.255.255.192 | 64 | 62 |
| /27 | 255.255.255.224 | 32 | 30 |
| /28 | 255.255.255.240 | 16 | 14 |
| /29 | 255.255.255.248 | 8 | 6 |
| /30 | 255.255.255.252 | 4 | 2 |
| /31 | 255.255.255.254 | 2 | 2 on an RFC 3021 point-to-point link |
| /32 | 255.255.255.255 | 1 | 1 address |
These are arithmetic counts, not provider-specific availability. Cloud platforms, managed networks, gateways, reservations, and internal policy may leave fewer addresses available.
Equal Splitting Is Not VLSM
The control offers 2, 4, 8, or 16 equal blocks. Each extra bit doubles the number of children and halves the addresses in each. A /30 can split into four /32 blocks, but not eight: there are only two host bits left.
A variable-length subnet-mask plan assigns differently sized children to different needs. This tool does not automatically pack those requirements, detect overlaps with your other networks, or decide which allocation you own.
Before You Change a Network
- Confirm the parent prefix. Use the actual allocation or interface configuration, not the example next to your public IP.
- Inventory what needs an address. Include gateways, infrastructure, static devices, reservations, and growth.
- Check for overlap. Include VPNs, remote sites, cloud networks, and existing routes.
- Plan each dependent change. Review DHCP, routing, VLAN interfaces, access controls, monitoring, and reverse DNS as relevant.
- Keep a rollback and management path. Avoid losing remote access while changing the interface you depend on.
- Verify with actual devices. A clean calculation is not a connectivity test or change approval.
Downloaded summaries contain the entered address and derived network plan. Share only the detail appropriate for the recipient.
Reference: RFC 4632 — CIDR address blocks and aggregation.