ZyXEL VMG3927-B50B User Guide - Page 91

Multicast, DNS Server Address Assignment

Page 91 highlights

Chapter 6 Broadband be created statically by hand or dynamically through GVRP. The VLAN ID associates a frame with a specific VLAN and provides the information that switches need to process the frame across the network. A tagged frame is four bytes longer than an untagged frame and contains two bytes of TPID (Tag Protocol Identifier), residing within the type/length field of the Ethernet frame) and two bytes of TCI (Tag Control Information), starts after the source address field of the Ethernet frame). The CFI (Canonical Format Indicator) is a single-bit flag, always set to zero for Ethernet switches. If a frame received at an Ethernet port has a CFI set to 1, then that frame should not be forwarded as it is to an untagged port. The remaining twelve bits define the VLAN ID, giving a possible maximum number of 4,096 VLANs. Note that user priority and VLAN ID are independent of each other. A frame with VID (VLAN Identifier) of null (0) is called a priority frame, meaning that only the priority level is significant and the default VID of the ingress port is given as the VID of the frame. Of the 4096 possible VIDs, a VID of 0 is used to identify priority frames and value 4095 (FFF) is reserved, so the maximum possible VLAN configurations are 4,094. TPID 2 Bytes User Priority 3 Bits CFI VLAN ID 1 Bit 12 Bits Multicast IP packets are transmitted in either one of two ways - Unicast (1 sender - 1 recipient) or Broadcast (1 sender - everybody on the network). Multicast delivers IP packets to a group of hosts on the network not everybody and not just 1. 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. 1 The ISP tells you the DNS server addresses, usually in the form of an information sheet, when you sign up. If your ISP gives you DNS server addresses, manually enter them in the DNS server fields. 2 If your ISP dynamically assigns the DNS server IP addresses (along with the VMG's WAN IP address), set the DNS server fields to get the DNS server address from the ISP. VMG4927-B50A / VMG9827-B50A / VMG3927-B50B User's Guide 91

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Chapter 6 Broadband
VMG4927-B50A / VMG9827-B50A / VMG3927-B50B User’s Guide
91
be created statically by hand or dynamically through GVRP. The VLAN ID associates a frame with a
specific VLAN and provides the information that switches need to process the frame across the network.
A tagged frame is four bytes longer than an untagged frame and contains two bytes of TPID (Tag
Protocol Identifier), residing within the type/length field of the Ethernet frame) and two bytes of TCI (Tag
Control Information), starts after the source address field of the Ethernet frame).
The CFI (Canonical Format Indicator) is a single-bit flag, always set to zero for Ethernet switches. If a
frame received at an Ethernet port has a CFI set to 1, then that frame should not be forwarded as it is to
an untagged port. The remaining twelve bits define the VLAN ID, giving a possible maximum number of
4,096 VLANs. Note that user priority and VLAN ID are independent of each other. A frame with VID
(VLAN Identifier) of null (0) is called a priority frame, meaning that only the priority level is significant and
the default VID of the ingress port is given as the VID of the frame. Of the 4096 possible VIDs, a VID of 0 is
used to identify priority frames and value 4095 (FFF) is reserved, so the maximum possible VLAN
configurations are 4,094.
Multicast
IP packets are transmitted in either one of two ways - Unicast (1 sender - 1 recipient) or Broadcast (1
sender - everybody on the network). Multicast delivers IP packets to a group of hosts on the network -
not everybody and not just 1.
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.
1
The ISP tells you the DNS server addresses, usually in the form of an information sheet, when you sign up.
If your ISP gives you DNS server addresses, manually enter them in the DNS server fields.
2
If your ISP dynamically assigns the DNS server IP addresses (along with the VMG’s WAN IP address), set
the DNS server fields to get the DNS server address from the ISP.
TPID
2 Bytes
User Priority
3 Bits
CFI
1 Bit
VLAN ID
12 Bits