X8821e/X8821m / X8824e/X8824m User’s Guide
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sequencing rules and supports re-keying
mechanisms.
Triggers
Triggers are used to deal with application
protocols that create separate sessions. Some
applications, such as NetMeeting, open
secondary connections during normal operations,
for example, a connection to a server is
established using one port, but data transfers are
performed on a separate connection. A trigger
tells the device to expect these secondary
sessions and how to handle them.
Once you set a trigger, the embedded IP address
of each incoming packet is replaced by the correct
host address so that NAT can translate packets to
the correct destination. You can specify whether
you want to carry out address replacement, and if
so, whether to replace addresses on TCP packets
only, UDP packets only, or both.
Twisted pair
The ordinary copper telephone wiring used by
telephone companies. It contains one or more
wire pairs twisted together to reduce inductance
and noise. Each telephone line uses one pair. In
homes, it is most often installed with two pairs.
For Ethernet LANs, a higher grade called
Category 3 (CAT 3) is used for 10BASE-T
networks, and an even higher grade called
Category 5 (CAT 5) is used for 100BASE-T
networks. See 10BASE-T, 100BASE-T, Ethernet.
Unnumbered
interfaces
An unnumbered interface is an IP interface that
does not have a local subnet associated with it.
Instead, it uses a router-id that serves as the
source and destination address of packets sent to
and from the router. Unlike the IP address of a
normal interface, the router-id of an unnumbered
interface is allowed to be the same as the IP
address of another interface. For example, the
WAN unnumbered interface of your device uses
the same IP address of the LAN interface
(192.168.1.1).
The unnumbered interface is temporary – PPP or
DHCP will assign a ‘real’ IP address
automatically.
Upstream
The direction of data transmission from the user to
the Internet.
VC Virtual
Circuit
A connection from your DSL router to your ISP.
VCI
Virtual Circuit Identifier
Together with the Virtual Path Identifier (VPI), the
VCI uniquely identifies a VC. Your ISP will tell you
the VCI for each VC they provide. See VC.