IP Multicast Introduction
What is the IP Multicast?
Traditionally, the IP packets are transmitted in two ways: unicast or broadcast.
Multicast is a third way to deliver the IP packets to a group of hosts. Host groups are
identified by class D IP addresses, i.e., those with "1110" as their higher-order bits. In
dotted decimal notation, host group addresses range from 224.0.0.0 to
239.255.255.255. Among them, 224.0.0.1 is assigned to the permanent IP hosts
group, and 224.0.0.2 is assigned to the multicast routers group.
The IGMP (Internet Group Management Protocol) is the protocol used to support
multicast groups. The latest version is version 2 (See RFC2236). The IP hosts use
IGMP to report their multicast group membership to any immediate-neighbor
multicast routers, so the multicast routers can decide if a multicast packet needs to
be forwarded. At the start-up, the Prestige queries all directly connected networks to
gather group membership.
After that, the CPE updates the information by periodic queries. The device
implementation of IGMP is also compatible with version 1. The multicast setting can
be turned on or off on the Ethernet and remote nodes.
NAT Introduction
What is NAT?
NAT (Network Address Translation-NAT RFC 1631) is the translation of an
Internet Protocol address used within one network to a different IP address known
within another network. One network is designated as the
inside
network and the
other is the
outside
. Typically, a company maps its local inside network addresses to
one or more global outside IP addresses and "unmaps" the global IP addresses on
the incoming packets back into local IP addresses. The IP addresses for NAT can be
either fixed or dynamically assigned by the ISP. In addition, you can designate servers,
e.g., a Web server and a Telnet server, on your local network and make them
accessible to the outside world. If you do not define any servers, the NAT offers the
additional benefit of firewall protection. In such case, all incoming connections to
