TCP/IP Tutorial and Technical Overview - IBM Redbooks

3.3 Internet Group Management Protocol (IGMP)
IGMP is a standard protocol with STD number 5. That standard also includes IP
(see 3.1, “Internet Protocol (IP)” on page 68) and ICMP (see 3.2, “Internet
Control Message Protocol (ICMP)” on page 109). Its status is recommended. It is
described in RFC 1112 with updates in RFC 2236.
Similar to ICMP, the Internet Group Management Protocol (IGMP) is also an
integral part of IP. It allows hosts to participate in IP multicasts. IGMP further
provides routers with the capability to check if any hosts on a local subnet are
interested in a particular multicast.
Refer to 6.2, “Internet Group Management Protocol (IGMP)” on page 241 for a
detailed review of IGMP.
3.4 Address Resolution Protocol (ARP)
3.4.1 ARP overview
Address Resolution Protocol (ARP) is a network-specific standard protocol. The
address resolution protocol is responsible for converting the higher-level protocol
addresses (IP addresses) to physical network addresses. It is described in
RFC 826.
On a single physical network, individual hosts are known in the network by their
physical hardware address. Higher-level protocols address destination hosts in
the form of a symbolic address (IP address in this case). When such a protocol
wants to send a datagram to destination IP address w.x.y.z, the device driver
does not understand this address.
Therefore, a module (ARP) is provided that will translate the IP address to the
physical address of the destination host. It uses a lookup table (sometimes
referred to as the ARP cache) to perform this translation.
When the address is not found in the ARP cache, a broadcast is sent out in the
network with a special format called the ARP request. If one of the machines in
the network recognizes its own IP address in the request, it will send an ARP
reply back to the requesting host. The reply will contain the physical hardware
address of the host and source route information (if the packet has crossed
bridges on its path). Both this address and the source route information are
stored in the ARP cache of the requesting host. All subsequent datagrams to this
destination IP address can now be translated to a physical address, which is
used by the device driver to send out the datagram in the network.
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