NAT / PAT Exhaustion
Model how many concurrent sessions your NAT/PAT can hold from public IPs and the ephemeral port range, how many users it supports and when it runs out. Simplified model (the real limit depends on the 5-tuple).
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Port utilization
Rule of thumb: keep utilization below 80 % to leave headroom for peaks and bursts.
Breakdown
| Item | Value | Note |
|---|
How it is calculated · and why it is an upper bound
1. Ports per IP = end_port − start_port + 1.
The typical ephemeral range is 1024–65535 = 64,512 ports.
2. Capacity = public_IPs × ports_per_IP.
It is the ceiling of concurrent outbound sessions the PAT can translate at once.
3. Demand = users × sessions_per_user.
Sessions per public IP = demand / IPs.
4. Utilization = demand / capacity × 100.
Exhaustion happens when demand exceeds capacity. Users supported =
capacity / sessions_per_user. IPs needed =
ceil(demand / ports_per_IP).
Note: this is an upper bound. In practice the NAT distinguishes sessions by the 5-tuple (source and destination IP and port + protocol), so the same source port can be reused toward different destinations. Real exhaustion depends on how many sessions go to the same destination and on the device's translation table.
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