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P2302R-P1C Support Notes
All contents Copyright 2007 ZyXEL Communications Corporation.
102
What is SUA? When should I use SUA?
SUA (Single User Account) is a unique feature supported by the ZyXEL Device which allows multiple people
to access Internet concurrently for the cost of a single user account.
When The ZyXEL Device acting as SUA receives a packet from a local client destined for the outside Internet,
it replaces the source address in the IP packet header with its own address and the source port in the TCP or
UDP header with another value chosen out of a local pool. It then recomputed the appropriate header
checksums and forwards the packet to the Internet as if it is originated from The ZyXEL Device using the IP
address assigned by ISP. When reply packets from the external Internet are received by The ZyXEL Device, the
original IP source address and TCP/UDP source port numbers are written into the destination fields of the
packet (since it is now moving in the opposite direction), the checksums are recomputed, and the packet is
delivered to its true destination. This is because SUA keeps a table of the IP addresses and port numbers of the
local systems currently using it.
What is the difference between NAT and SUA?
NAT is a generic name defined in RFC 1631 'The IP Network Address Translator (NAT)'. SUA (Internet
Single User Account) is ZyXEL's implementation and trade name for functioning PAT which is a specific type
of NAT. SUA (or PAT for NAT) translates address into port mapping.
The primary motivation for RFC 1631 is that there is not enough IP address to go around. In addition, many
corporations simply did not bother to obtain legal (globally unique) IP addresses for their networks and now
finding themselves unable to connect to the Internet.
Basically, NAT is a process of translating one address to another. A NAT implementation can be as simple as
substituting an IP address with another. This allows a network to rectify the illegal address problem mentioned
above without going through each and every host.
The design goal of ZyXEL's SUA is to minimize the Internet access cost in a small office environment by using
a single IP address to represent the multiple hosts inside. It does more than IP address translation, so that
multiple hosts on the LAN can access the Internet at the same time.
How many network users can the SUA/NAT support?
The ZyXEL Device does not limit the number of the users but the number of the sessions. The ZyXEL Device
supports 1024 sessions that you can use the 'ip nat iface enif0 disp' command in menu 24.8 to view the current
active sessions.