Reference Manual for the Model RP614 Web Safe Router
Network and Routing Basics B-9
This scheme offers the additional benefit of si mple firewall-like protection because the int ernal
LAN addresses are not available to the Intern et through the translated connection. All inc oming
inquiriesare filtered out by the router. This filtering can preventintruders from probing your
system.However, using port forwarding, you can allow one PC (for example, a Webserver) on
your localnetwork to be accessible to outside users.
MAC Addresses and Address Resolution Protocol
An IP address alonecannot be used to deliver data from one LAN device to another. To send data
between LANdevices, you must convert the IP address of the destination device to its media
accesscontrol (MAC) address. Each device on an Ethernet network has a unique MAC address,
which is a 48-bit number assignedto each device by the manufacturer. The technique that
associatesthe IP address with a MAC address is known as address resolution. Internet Protocol
uses the AddressResolution Protocol (ARP) to resolve MAC addresses.
Ifa device sends data to another station on the network and the destination MAC address is not yet
recorded,ARP is used. An ARP request is broadcast onto the network. All stations on the network
receiveand read the request. The destination IP address fort hechosen station is included as part of
the messageso that only the station with this IP address responds to the ARP request. All other
stationsdiscard the request.
The stationwith the correct IP address responds with its own MAC address directly to the sending
device.The receiving station provides the transmitting station with the required destination MAC
address.The IP address data and MAC address data for each station are held in an ARP table. The
next time datais sent, the address can be obtained from the address information in the table.
Domain Name Server
Many of the resourceson the Internet can be addressed by simple descriptive names such as
www.NETGEAR.com.This addressing is very helpful at the application level, but the descriptive
namemust be translated to an IP address in order for a user to actually contact the resource. Just as
a telephonedirectory maps names to phone numbers, or as an ARP table maps IP addresses to
MAC addresses,a domain name system(DNS) server maps descriptivenames of network
resourcesto IP addresses.
Whena PC accessesa resource by its descriptive name, it first contacts a DNS server to obtain the
IP addressof the resource. The PC sends the desired message using the IP address. Manylarge
organizations,such as ISPs, maintain their own DNS servers and allow their customers to usethe
servers to look up addresses.