Specifications

AVM Access Server – Glossary 131
NAT (Network Address Translation)
NAT is a technique in which a router replaces addresses and port num-
bers in IP, UDP and TCP packet headers with other values. The AVM
Access Server performs NAT using a table to map the original IP ad-
dress and port numbers to new values. For incoming connections han-
dled by a forwarding profile, this table is static. Outgoing connections
are handled dynamically by IP masquerading.
IP masquerading and forwarding profiles are special uses of NAT.
In IP masquerading, the source IP addresses in outgoing TCP, UDP and IC-
MP packets are replaced with the AVM Access Server’s current public IP
address. Conversely, the destination address in replies to these packets
arriving from the Internet is replaced with the IP address of the request-
ing client in the LAN. In this way the LAN appears in the Internet only as a
single public IP address. IP masquerading is also called source NAT.
Forwarding profiles are used to replace the destination address in re-
quest packets arriving from the Internet—that is, AVM Access Server’s
public IP address—with the internal address of an appropriate server in
the LAN. In this way the AVM Access Server can forward incoming e-mail,
for example, to a specific SMTP server in the private LAN, even if the con-
nection to the Internet uses a single dynamically assigned IP address.
This form of NAT is also called destination NAT or port forwarding.
NetBIOS
A standard for network communication that is independent of underly-
ing transport protocols. NetBIOS is the standard network interface in
Microsoft networks, and can be transported over both IP and IPX. Net-
BIOS uses numerous broadcasts, which can be intercepted by the AVM
Access Server’s special filter to reduce connection costs.
Network address: see “IP address” on page 127
Outside dialing prefix
The outside dialing prefix is the digit that must be dialed on a PBX ex-
tension line before dialing a number on the public telephone network.
In modern PBX systems this is usually “0”. In the AVM Access Server,
the outside dialing prefix can be specified for each ISDN-Controller in-
dividually (on the “General” dialog page in the folder “Administration /
Interfaces”). The Access Server then uses the outside dialing prefix au-
tomatically where appropriate.