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Chapter 6: Glossary
145
Chapter 6: Glossary
ARP (Address Resolution Protocol )
ARP is a TCP/IP protocol for mapping an IP address to a physical machine address that is
recognized in the local network, such as an Ethernet address.
A host wishing to obtain a physical address broadcasts an ARP request onto the TCP/IP network.
The host on the network that has the IP address in the request then replies with its physical
hardware address.
Inverse ARP (In-ARP), on the other hand, is used by a host to discover its IP address. In this case,
the host broadcasts its physical address and a RARP server replies with the host's IP address.
DHCP (Dynamic Host Configuration Protocol)
When operates as a DHCP server, the ADSL Router assign IP addresses to the client PCs on the
LAN. The client PCs “leases” these Private IP addresses for a user-defined amount of time. After
the lease time expires, the private IP address is made available for assigning to other network
devices.
The DHCP IP address can be a single, fixed public IP address, an ISP assigned public IP address,
or a private IP address.
If you enable DHCP server on a private IP address, a public IP address will have to be assigned
to the NAT IP address, and NAT has to be enabled so that the DHCP IP address can be
translated into a public IP address. By this, the client PCs are able to access the Internet.
DSCP - Differentiated Services (DiffServ) Code Point
A Differentiated Services (DiffServ) Code Point is a marker in the header of each IP packet that
prompts network routers to apply differentiated grades of service to various packet streams. It is
specified by the DiffServ policy proposed by the
IETF (Internet Engineering Task Force). This
allows Internet and other IP-based network service providers to offer different levels of service to
customers.
LAN (Local Area Network) & WAN (Wide Area Network)
A LAN is a computer network limited to the immediate area, usually the same building or floor of a
building. A WAN, on the other hand, is an outside connection to another network or the Internet.
The Ethernet side of the ADSL Router is called the LAN port. It is a twisted-pair Ethernet
10Base-T interface. A hub can be connected to the LAN port. More than one computers, such as
server or printer, can be connected through this hub to the ADSL Router and composes a LAN.
The DSL port of the ADSL Router composes the WAN interface, which supports PPP or RFC
1483 connecting to another remote DSL device.
NAT (Network Address Translation) IP Address
NAT is an Internet standard that translates a private IP within one network to a public IP address,
either a static or dynamic one. NAT provides a type of firewall by hiding internal IP addresses. It
also enables a company to use more internal IP addresses.
If the IP addresses given by your ISP are not enough for each PC on the LAN and the ADSL
Router, you need to use NAT. With NAT, you make up a private IP network for the LAN and
assign an IP address from that network to each PC. One of some public addresses is configured
and mapped to a private workstation address when accesses are made through the gateway to a
public network.
For example, the ADSL Router is assigned with the public IP address of 168.111.2.1. With NAT
enabled, it creates a Virtual LAN. Each PC on the Virtual LAN is assigned with a private IP
address with default value of 192.168.2.2 to 192.168.2.254. These PCs are not accessible by the
outside world but they can communicate with the outside world through the public IP 168.111.2.1.