CyberGuard dictionary / Network Security
Carrier Grade NAT
Carrier-grade NAT is address and port translation operated by an internet provider so multiple customers can share public IPv4 addresses.
In simple terms
A household router may do NAT, and the provider may add another translation layer before traffic reaches the Internet.
Technical explanation
Providers use CGN to conserve public IPv4 space; RFC 6598 reserves 100.64.0.0/10 as shared address space for this purpose. Shared public addresses complicate inbound connections and attribution without port and time records.
A service provider can share one public IPv4 address among many subscribers by translating their connections. Attribution needs precise time and port information, and inbound hosting can be difficult.
Examples in practice
Two customers appear to a website under the same public IPv4 address at different source ports and times.
A service log records one public address that belongs to many households. Investigators avoid assigning the event to a subscriber until provider records can match the source port and timestamp.
Why it matters
Understanding CGN helps explain remote-access problems and why an IP address alone may not identify one subscriber.
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Sources and further reading
- www.rfc-editor.orghttps://www.rfc-editor.org/info/rfc6598/↗
- www.rfc-editor.orghttps://www.rfc-editor.org/info/rfc3022/↗
Published 2026-10-15 · Last updated 2026-10-15 · CyberGuard editorial content. Source links provide context; they do not imply endorsement.