Chapter 6 Broadband
VMG1312-B Series User’s Guide
105
Multicast
I P packet s are t ransm itt ed in eit her one of two ways - Unicast (1 sender - 1 recipient ) or Broadcast
( 1 sender - everybody on the network) . Mult icast delivers I P packet s t o a group of hosts on t he
net work - not everybody and not just 1.
I nternet Group Mult icast Prot ocol ( I GMP) is a network- layer prot ocol used t o establish m em bership
in a Mult icast group - it is not used t o carry user dat a. I GMP version 2 ( RFC 2236) is an
im provem ent over version 1 (RFC 1112) but I GMP version 1 is st ill in wide use. I f you would like to
read m ore detailed inform ation about int eroperabilit y between I GMP version 2 and version 1, please
see sect ions 4 and 5 of RFC 2236. The class D I P address is used to identify host groups and can be
in t he 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 I P m ulticast com puters. The address 224.0.0.1 is used for query m essages and is
assigned t o the perm anent group of all I P hosts ( including gat eways). All hosts m ust j oin the
224.0.0.1 group in order t o participat e in I GMP. The address 224.0.0.2 is assigned t o t he m ult icast
routers group.
At st art up, the Device queries all direct ly connected networks t o gather group m em bership. After
that, the Device periodically updat es t his inform at ion.
DNS Server Address Assignment
Use Dom ain Nam e System ( DNS) to m ap a dom ain nam e to its corresponding I P addr ess and vice
versa, for instance, t he I P address of www.zyxel.com is 204.217.0.2. The DNS server is extrem ely
im portant because without it, you m ust know t he I P address of a com put er before you can access
it.
The Device can get the DNS server addresses in t he following ways.
1 The I SP tells you t he DNS server addresses, usually in t he form of an inform ation sheet , when you
sign up. I f your I SP gives you DNS server addresses, m anually ent er t hem in the DNS server fields.
2 I f your I SP dynam ically assigns the DNS server I P addresses (along wit h the Device’s WAN I P
address) , set t he DNS server fields t o get the DNS server address from t he I SP.
IPv6 Addressing
The 128- bit I Pv6 address is writ t en as eight 16- bit hexadecim al blocks separated by colons ( : ) . This
is an exam ple I Pv6 address 2001:0db8:1a2b:0015:0000:0000:1a2f:0000.
I Pv6 addresses can be abbreviat ed in t wo ways:
• Leading zeros in a block can be om it ted. So 2001:0db8:1a2b:0015:0000:0000:1a2f:0000 can
be writt en as 2001:db8:1a2b:15:0:0:1a2f:0.
• Any num ber of consecutive blocks of zeros can be replaced by a double colon. A double colon can
only appear once in an I Pv6 address. So 2001:0db8:0000:0000:1a2f:0000:0000:0015 can be
writ t en 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
Sim ilar t o an I Pv4 subnet m ask, I Pv6 uses an address prefix t o represent the network address. An
I Pv6 prefix length specifies how m any m ost significant bit s ( start from t he left) in the address
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