ipThe role of address translation
A home router can send traffic from privately addressed devices through a public IPv4 address. Common setups track connections using port information as well. Returning traffic is mapped back to the correct device, reducing the need for a separate public IPv4 address for each device.
Outbound and inbound differ
A browser-initiated connection creates a mapping on the router. For a new connection initiated externally, the intended internal device may be unknown. Port forwarding can define that destination in some cases, but it does not automatically bypass an additional NAT layer at the ISP.
How it differs from a firewall
NAT concerns address translation and connection mappings; a firewall enforces traffic-allowance policy. They can operate on the same device without being synonymous. Before exposing a service, separately consider authentication, updates, and restricting access to the sources that actually need it.
Two browsers behind one IP
When two home devices visit a site, the router can distinguish connections through address and port mappings. The site may see the same source IPv4 on both requests. Counting unique IPs therefore does not reliably count users. Session measurement requires separate application-level methods assessed against the relevant privacy requirements.
Further reading
Traditional NAT — RFC 3022 ↗Put it into practice
Inspect one public IP, or compare a list of addresses with the bulk lookup tool. Location results are approximate.
Single lookup ↗ Bulk lookup ↗