Bay Technical Associates RM356 manual Address Resolution Protocol, Domain Name Server

Reference Guide for the Model RM356 Modem Router

This scheme offers the additional benefit of firewall protection because the internal LAN addresses are not available to the Internet through the translated connection. All incoming inquiries are filtered out by the router. This filtering can prevent intruders from probing your system. You can specify one server (for example, a Web server) on your local network and make it accessible by outside users.

Address Resolution Protocol

An IP address alone cannot be used to deliver data from one device to another on a LAN. In order for data to be sent from one device on the LAN to another, you must convert the IP address of the destination device to its media access control (MAC) address. Each device on an Ethernet network has a unique Ethernet MAC address, which is a 48-bit number assigned to each device by the manufacturer. The technique that associates the IP address with a MAC address is known as address resolution, and IP uses the Address Resolution Protocol (ARP) to do this.

If a device needs to send data to another station on the network and it does not already have the destination MAC address recorded, ARP is used. An ARP request is broadcast onto the network, and all stations receive and read the request. The destination IP address for the chosen station is included as part of the message so that only the station with this IP address responds to the ARP request and all other nodes discard it.

The node with the right IP address responds with its own MAC address directly to the sender, providing the transmitting station with the destination MAC address needed for it to send the data. The IP address data and MAC address data for each node are held in an ARP table, so that the next time data needs to be sent, the address can be obtained from the address information in the table.

Domain Name Server

Many of the resources on the Internet can be addressed by simple descriptive names such as www.baynetworks.com. This addressing is very helpful at the application level, but the descriptive name must be translated to an IP address in order for a user to actually contact the resource. Just as each workstation maintains an ARP table to map IP addresses to MAC addresses, a domain name server (DNS) maps descriptive names of network resources to IP addresses.

When a workstation needs to access a resource by its descriptive name, it first contacts a DNS to obtain the IP address of the resource. It can then send the desired message using the IP address. Many large organizations such as ISPs maintain their own DNSs and allow their customers to use them for address lookup.

Introduction 1-13