Understanding the network.pdf - Back to Home

uses both TCP and UDP for service delivery. TCP port 53 is used for server-to-server
data transfers. The data consists of the forward (hostname to IP address) and
reverse (IP address to hostname) translation files. Name requests are performed
using UDP.
RARP, BOOTP, and DHCP
In order for a host to exchange data with other hosts, each host must have its own
distinct IP address. Reverse Address Resolution Protocol (RARP) is the simplest way
to provide IP addresses to hosts. The server listens to broadcasts sent across the
Ethernet segment. If it receives a broadcast from a host with an Ethernet address
that it knows, it responds to the broadcast with the IP address of the requesting host.
This is great if all that's needed is an IP address, and if the host and server are on
the same Ethernet segment. RARP's functional shortcomings leave it little place in
today's networks.
BOOTP, on the other hand, picks up where RARP leaves off. BOOTP uses IP to
provide IP configuration and boot service, hence the name BOOTP, for
bootstrapping protocol. The requesting host sends a BOOTP request, which is either
answered by a BOOTP server that has a configuration for the requesting station, or
the request is relayed to a BOOTP server on another Ethernet segment that contains
the host configuration information.
The BOOTP server runs as a UDP service on port 68. Although there is no interactive
authentication, hosts must be configured in the BOOTP server database and have
corresponding BOOTP images in the TFTP server for the BOOTP service to work.
After the configuration information has been transferred to the host, the BOOTP
server then directs the host to the TFTP server where the actual boot file resides.
This boot file is then downloaded through TFTP and used to boot the host. BOOTP
provides more services than RARP, so the two cannot be compared directly.
BOOTP's capability to provide all the network configuration information (default
gateway, DNS servers, and so on), its capability to boot the host from a remote
server, and its capability to have service requests relayed across separate Ethernet
cable segments make BOOTP more versatile than RARP. In terms of providing IP
addresses to hosts, BOOTP is a better choice because it uses IP and does not have
the direct hardware access requirement RARP has. BOOTP does not work well in
environments that are constantly changing (in other words, lots of laptops) and
where configuration information needs to be changed on a constant basis.
Dynamic Host Configuration Protocol (DHCP) provides IP host configuration services
dynamically with no host configuration requirements. To use DHCP, the host's IP
implementation must support configuration by DHCP. As with BOOTP, the host
makes a DHCP broadcast request and the DHCP server replies with configuration