How Many Public IP Addresses Does a WISP Tower Need?
A practical guide to subnet sizing, customer counts, CGNAT and public IPv4 planning per tower.
One of the most common questions we hear from growing WISPs is: "How many public IP addresses do I actually need per tower?" The answer depends on your subscriber mix, whether you use Carrier-Grade NAT (CGNAT), and how much headroom you want for business customers.
This guide gives you a practical framework for sizing public IPv4 blocks per tower, including the math, common mistakes, and when to ask for a larger block than you think you need.
Start with the subscriber mix
Not every subscriber needs a public IP. Most residential customers work fine behind CGNAT. Business customers, VoIP users, gamers, and customers with VPNs or surveillance systems often need a public IP.
- Residential/CGNAT: 0 public IPs per subscriber.
- Residential with public IP: 1 public IP per subscriber.
- Small business: 1–2 public IPs.
- VoIP or hosted PBX: 1 public IP per phone system or site.
- Cameras / remote access: 1 public IP per site or device group.
Reserve infrastructure IPs
Every routed subnet wastes some addresses. You need to reserve space for:
- Network address
- Broadcast address
- Default gateway for the tower router
- Backup or redundant router
- Management interfaces (switches, OOB, etc.)
- Spare addresses for growth
A good rule of thumb is to reserve 5–10 infrastructure IPs per tower, depending on complexity.
Quick sizing formula
Then round up to the next standard block size: /30 (2 usable), /29 (6), /28 (14), /27 (30), /26 (62), /25 (126), /24 (254).
Example towers
| Subscribers | Public IP ratio | Usable IPs needed | Suggested block |
|---|---|---|---|
| 100 | 10% | ~20 | /27 (30 usable) |
| 250 | 20% | ~60 | /26 (62 usable) |
| 500 | 30% | ~160 | /24 (254 usable) |
Common mistakes
- Forgetting broadcast and gateway addresses. A /29 only gives 6 usable IPs, not 8.
- No headroom for business growth. Once a /29 is full, renumbering is painful.
- Putting CGNAT and public-IP customers in the same subnet. Keep these logically separate for troubleshooting.
- Using a separate /30 for every small need. This wastes blocks and complicates routing.
Need help with IP design?
SmashByte Wireless helps WISPs size IP blocks, design subnet plans, and secure BGP announcements. Use our calculator or talk to an expert.