ZyXEL Communications SBG Series Support Notes page 25

ZyXEL – SBG Support Notes
Internal Local Addresses (ILA) and the global IP addresses as the Inside
Global Addresses (IGA). The term 'inside' refers to the set of networks that
are subject to translation. The NAT operates by mapping the ILA to the IGA
required for communication with hosts on other networks. It replaces the
original IP source address (and TCP or UDP source port numbers) and then
forwards each packet to the Internet ISP, thus making the packets appear
as if they came from the NAT system itself (e.g., the CPE router). The CPE
keeps track of the original addresses and port numbers, so the incoming
reply packets can have their original values restored.
What is 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.
What is BOOTP/DHCP?
BOOTP stands for Bootstrap Protocol. DHCP stands for Dynamic Host
Configuration Protocol. Both are mechanisms to dynamically assign an IP
address to a TCP/IP client by the server. In this case, the device is a
BOOTP/DHCP server. Windows 95 and Windows NT clients use DHCP to
request an internal IP address, while WFW and WinSock clients use
BOOTP. TCP/IP clients may specify their own IP or utilize
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