CGNAT Calculator
Stretch a scarce IPv4 pool across thousands of subscribers. Size port blocks, see subscribers per public IP and how many addresses Carrier-Grade NAT actually saves you.
NAT Pool Parameters
/24 = 256 • /25 = 128 • /26 = 64
Common blocks: 512, 1024, 2048
65535 total, minus reserved (~64512)
Pool Capacity
Healthy headroom- Subscribers per public IP
- 63
- Public IPs needed
- 80
- IPv4 addresses saved
- 4,872
- Contention ratio
- 39:1
Manage the IPs behind the NAT.
Built-in IPAM tracks public pools, CGNAT ranges and per-subscriber allocations. Free to start.
Create free accountHow CGNAT (Carrier-Grade NAT) stretches IPv4
With IPv4 exhausted and address blocks costing real money, most growing ISPs put subscribers behind Carrier-Grade NAT (CGNAT / NAT444 / LSN). Instead of one public IPv4 per customer, many customers share a single public address, each assigned a fixed block of TCP/UDP ports.
The math
- Subscribers per public IP = usable ports per IP ÷ ports per subscriber.
- Pool capacity = public IPs × subscribers per IP.
- Ports per subscriber is the trade-off: smaller blocks pack more subscribers per IP, but too few ports break port-hungry apps (browsers, streaming, gaming).
Choosing a port block
Common allocations run from 256 to 2048 ports per subscriber. 1024 is a widely used balance. Reserve the low/well-known port range, so plan on roughly 64512 usable ports per public IP per protocol. Keep pool utilisation under ~85% so you have headroom for peaks before you have to buy more IPv4 or push IPv6.
