Advanced IPv4 Subnet Tool
IPv4 Subnet Calculator
Enter any IPv4 address and prefix or subnet mask — get full subnet details, binary breakdown, wildcard mask, VLSM planning, and subnet splitting with Cisco IOS export.
| # | Subnet / Prefix | Subnet Mask | Network Addr | Broadcast | First Host | Last Host | Usable Hosts | Label / Purpose |
|---|
| Order | Label | Hosts Needed | Prefix Assigned | Subnet Mask | Network Addr | Broadcast | First Host | Last Host | Usable Hosts | Remove |
|---|
| CIDR | Subnet Mask | Wildcard | Total Addresses | Usable Hosts | Typical Use |
|---|
| Range | CIDR | Addresses | RFC | Common Use |
|---|---|---|---|---|
| 10.0.0.0 – 10.255.255.255 | 10.0.0.0/8 | 16,777,216 | RFC 1918 | Large enterprise / data centre / supernet |
| 172.16.0.0 – 172.31.255.255 | 172.16.0.0/12 | 1,048,576 | RFC 1918 | Medium enterprise, ISP-internal |
| 192.168.0.0 – 192.168.255.255 | 192.168.0.0/16 | 65,536 | RFC 1918 | SOHO, lab, home networking |
| 100.64.0.0 – 100.127.255.255 | 100.64.0.0/10 | 4,194,304 | RFC 6598 | Carrier-grade NAT (CGN / LSN) |
| 169.254.0.0 – 169.254.255.255 | 169.254.0.0/16 | 65,536 | RFC 3927 | APIPA / link-local (no DHCP found) |
| Address / Range | Type | RFC | Notes |
|---|---|---|---|
| 0.0.0.0/8 | This network (unspecified) | RFC 1122 | Source address during DHCP discovery |
| 127.0.0.0/8 | Loopback | RFC 1122 | 127.0.0.1 is commonly used; full /8 is reserved |
| 192.0.0.0/24 | IETF Protocol Assignments | RFC 6890 | Reserved for IETF protocol purposes |
| 192.0.2.0/24 | TEST-NET-1 | RFC 5737 | Documentation and example use only |
| 198.51.100.0/24 | TEST-NET-2 | RFC 5737 | Documentation and example use only |
| 203.0.113.0/24 | TEST-NET-3 | RFC 5737 | Documentation and example use only |
| 224.0.0.0/4 | Multicast | RFC 1112 | 224.0.0.1 all hosts, 224.0.0.2 all routers |
| 240.0.0.0/4 | Reserved | RFC 1112 | Future use; not routable |
| 255.255.255.255/32 | Limited Broadcast | RFC 919 | Broadcast to all on local segment |
IPv4 Subnetting — Complete Reference for Network Engineers
This tool handles every IPv4 subnetting task: subnet mask lookup, wildcard mask, binary breakdown, usable host count, subnet splitting in three modes, VLSM planning, and Cisco IOS export. Use it for network design, exam prep (CCNA/CCNP), and troubleshooting.
What is CIDR / Prefix Notation?
Classless Inter-Domain Routing (CIDR) replaced class-based addressing. A /24 means 24 bits are network bits, giving subnet mask 255.255.255.0 and 254 usable hosts. The prefix length can be any value from /1 to /32.
Subnet Mask vs Wildcard Mask
The subnet mask has all network bits set to 1 and host bits set to 0 (e.g. 255.255.255.0). The wildcard mask is the bitwise inverse — all host bits are 1, all network bits are 0 (e.g. 0.0.0.255). Cisco IOS ACLs and OSPF use the wildcard mask.
Usable Hosts Formula
In any subnet: usable hosts = 2^(host bits) − 2. We subtract 2 because the all-zeros address is the network address and the all-ones address is the broadcast address, both reserved. Exception: /31 subnets per RFC 3021 have no reserved addresses and both addresses are usable on point-to-point links. /32 is a host route with exactly one address.
VLSM (Variable Length Subnet Masking)
VLSM lets you assign different prefix lengths to different subnets within the same address block, minimising waste. The VLSM Planner above allocates subnets largest-first from your parent block: enter each network's host requirement and the tool selects the smallest prefix that fits.
Point-to-Point Links — /30 and /31
/30 gives 4 addresses, 2 usable — the traditional choice for router-to-router links. /31 per RFC 3021 gives 2 addresses both usable, halving the waste. Cisco IOS supports /31 with ip address x.x.x.x 255.255.255.254.
Cisco IOS Quick Reference
Assign IP: ip address 192.168.1.1 255.255.255.0. Verify: show ip interface brief and show ip route. ACL with wildcard: access-list 10 permit 192.168.1.0 0.0.0.255. OSPF network: network 10.0.0.0 0.255.255.255 area 0.