ZyXEL Communications VMG3925-B30C User Manual page 98

Internet Group Multicast Protocol (IGMP) is a network-layer protocol used to establish membership in a
Multicast group - it is not used to carry user data. IGMP version 2 (RFC 2236) is an improvement over
version 1 (RFC 1112) but IGMP version 1 is still in wide use. If you would like to read more detailed
information about interoperability between IGMP version 2 and version 1, please see sections 4 and 5 of
RFC 2236. The class D IP address is used to identify host groups and can be in the range 224.0.0.0 to
239.255.255.255. The address 224.0.0.0 is not assigned to any group and is used by IP multicast
computers. The address 224.0.0.1 is used for query messages and is assigned to the permanent group of
all IP hosts (including gateways). All hosts must join the 224.0.0.1 group in order to participate in IGMP.
The address 224.0.0.2 is assigned to the multicast routers group.
At start up, the VMG queries all directly connected networks to gather group membership. After that,
the VMG periodically updates this information.
DNS Server Address Assignment
Use Domain Name System (DNS) to map a domain name to its corresponding IP address and vice versa,
for instance, the IP address of www.zyxel.com is 204.217.0.2. The DNS server is extremely important
because without it, you must know the IP address of a computer before you can access it.
The VMG can get the DNS server addresses in the following ways.
The ISP tells you the DNS server addresses, usually in the form of an information sheet, when you sign up.
1
If your ISP gives you DNS server addresses, manually enter them in the DNS server fields.
If your ISP dynamically assigns the DNS server IP addresses (along with the VMG's WAN IP address), set
2
the DNS server fields to get the DNS server address from the ISP.
IPv6 Addressing
The 128-bit IPv6 address is written as eight 16-bit hexadecimal blocks separated by colons (:). This is an
example IPv6 address 2001:0db8:1a2b:0015:0000:0000:1a2f:0000.
IPv6 addresses can be abbreviated in two ways:
• Leading zeros in a block can be omitted. So 2001:0db8:1a2b:0015:0000:0000:1a2f:0000 can be
written as 2001:db8:1a2b:15:0:0:1a2f:0.
• Any number of consecutive blocks of zeros can be replaced by a double colon. A double colon can
only appear once in an IPv6 address. So 2001:0db8:0000:0000:1a2f:0000:0000:0015 can be
written as 2001:0db8::1a2f:0000:0000:0015, 2001:0db8:0000:0000:1a2f::0015,
2001:db8::1a2f:0:0:15 or 2001:db8:0:0:1a2f::15.
IPv6 Prefix and Prefix Length
Similar to an IPv4 subnet mask, IPv6 uses an address prefix to represent the network address. An IPv6
prefix length specifies how many most significant bits (start from the left) in the address compose the
network address. The prefix length is written as "/x" where x is a number. For example,
2001:db8:1a2b:15::1a2f:0/32
means that the first 32 bits (2001:db8) is the subnet prefix.
Chapter 6 Broadband
VMG3925-B10C/B30C User's Guide
98