Operation Manual

AT-RG213 Residential VoIP Gateway - SIP Software Reference Manual

SIP Software Release 6-0-0

J613-M0524-00

Generally they would be allocated geographically with subnet 2 being one

site, subnet 3 another and so on. Some sites may have a requirement for

multiple subnets on the same LAN.

This could be to increase the number of hosts or simply to make

administration easier. In this case it is normal (but not required) that the

subnets be assigned contiguously for this site. This makes the allocation of a

subnet mask easier.

This mask is needed by the routers to ascertain which subnets are available

at each site. Bits in the mask are set to ‘1’ if the router is to treat the

corresponding bit in the IP address as belonging to the network portion or

set to ‘0’ if it belongs to the host portion. This allows a simple bit-wise logical

AND to determine if the address should be forwarded or not. Although the

standard does not require that the subnet mask must select contiguous bits,

it is normal practice to do so. To do otherwise can make the allocation of

numbers rather difficult and prone to errors. Some example masks are:

11111111.11111111.11111111.00000000 = 255.255.255.0

<----network--------> <subnet> <-host->

This would give 254 subnets on a class B network, each with 254 hosts.

11111111.11111111.11111111.11110000 = 255.255.255.240

<------network-----> <----subnet----><host>

This would give 4094 subnets on a class B network, each with 14 hosts or, 14

subnets on a class C network each with 14 hosts.

Multicasting, IGMP and IGMP snooping

What is Multicasting?

Multicasting is a technique developed to send packets from one location in

the Internet to many other locations, without any unnecessary packet

duplication. In multicasting, one packet is sent from a source and is

replicated as needed in the network to reach as many end-users as

necessary.

The concept of a group is crucial to multicasting. Every multicast requires a

multicast group; the sender (or source) transmits to the group address, and

only members of the group can receive the multicast data. A group is

defined by a Class D address.