NETGEAR DG824M Router Manual PDF (Setup & Configuration Guide)

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Reference Manual for the Model DG824M Wireless ADSL Modem Gateway

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Network and Routing Basics

Single IP Address Operation Using NAT

In the past, if multiple PCs on a LAN needed to access the Internet simultaneously, you had to

obtain a range of IP addresses from the ISP. This type of Internet account is more costly than a

single-address account typically used by a single user with a modem, rather than a router. The

DG824M gateway employs an address-sharing method called Network Address Translation

(NAT). This method allows several networked PCs to share an Internet account using only a single

IP address, which may be statically or dynamically assigned by your ISP.

The router accomplishes this address sharing by translating the internal LAN IP addresses to a

single address that is globally unique on the Internet. The internal LAN IP addresses can be either

private addresses or registered addresses. For more information about IP address translation, refer

to RFC 1631,

The IP Network Address Translator (NAT)

.

The following figure illustrates a single IP address operation.

Figure 7-3: Single IP Address Operation Using NAT

7786EA

192.168.0.2

192.168.0.3

192.168.0.4

192.168.0.5

192.168.0.1

172.21.15.105

Private IP addresses

assigned by user

Internet

IP addresses

assigned by ISP

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Reference Manual for the Model DG824M Wireless ADSL Modem Gateway

Network and Routing Basics

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This scheme offers the additional benefit of firewall-like 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. However, using port forwarding, you can allow one PC (for example, a Web server) on

your local network to be accessible to outside users.

MAC Addresses and Address Resolution Protocol

An IP address alone cannot be used to deliver data from one LAN device to another. To send data

between LAN devices, 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 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. Internet Protocol

uses the Address Resolution Protocol (ARP) to resolve MAC addresses.

If a 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

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. All other

stations discard the request.

Related Documents

The station with 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 data is sent, the address can be obtained from the address information in the table.

For more information about address assignment, refer to the IETF documents RFC 1597,

Address

Allocation for Private Internets,

and RFC 1466,

Guidelines for Management of IP Address Space

.

For more information about IP address translation, refer to RFC 1631,

The IP Network Address

Translator (NAT)

.

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Reference Manual for the Model DG824M Wireless ADSL Modem Gateway

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Network and Routing Basics

Domain Name Server

Many of the resources on 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

name must be translated to an IP address in order for a user to actually contact the resource. Just as

a telephone directory maps names to phone numbers, or as an ARP table maps IP addresses to

MAC addresses, a domain name system (DNS) server maps descriptive names of network

resources to IP addresses.

When a PC accesses a resource by its descriptive name, it first contacts a DNS server to obtain the

IP address of the resource. The PC sends the desired message using the IP address. Many large

organizations, such as ISPs, maintain their own DNS servers and allow their customers to use the

servers to look up addresses.

IP Configuration by DHCP

When an IP-based local area network is installed, each PC must be configured with an IP address.

If the PCs need to access the Internet, they should also be configured with a gateway address and

one or more DNS server addresses. As an alternative to manual configuration, there is a method by

which each PC on the network can automatically obtain this configuration information. A device

on the network may act as a Dynamic Host Configuration Protocol (DHCP) server. The DHCP

server stores a list or pool of IP addresses, along with other information (such as gateway and DNS

addresses) that it may assign to the other devices on the network. The DG824M gateway has the

capacity to act as a DHCP server.

The DG824M gateway also functions as a DHCP client when connecting to the ISP. The gateway

can automatically obtain an IP address, subnet mask, DNS server addresses, and a gateway address

if the ISP provides this information by DHCP.

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Reference Manual for the Model DG824M Wireless ADSL Modem Gateway

Network and Routing Basics

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Ethernet Cabling

Although Ethernet networks originally used thick or thin coaxial cable, most installations currently

use unshielded twisted pair (UTP) cabling. The UTP cable contains eight conductors, arranged in

four twisted pairs, and terminated with an RJ45 type connector. A normal "straight-through" UTP

Ethernet cable follows the EIA568B standard wiring as described in

Table 7-1

.

Uplink Switches and Crossover Cables

In the wiring table, the concept of transmit and receive are from the perspective of the PC. For

example, the PC transmits on pins 1 and 2. At the hub, the perspective is reversed, and the hub

receives on pins 1 and 2. When connecting a PC to a PC, or a hub port to another hub port, the

transmit pair must be exchanged with the receive pair. This exchange is done by one of two

mechanisms. Most hubs provide an Uplink switch which will exchange the pairs on one port,

allowing that port to be connected to another hub using a normal Ethernet cable. The second

method is to use a crossover cable, which is a special cable in which the transmit and receive pairs

are exchanged at one of the two cable connectors. Crossover cables are often unmarked as such,

and must be identified by comparing the two connectors. Since the cable connectors are clear

plastic, it is easy to place them side by side and view the order of the wire colors on each. On a

straight-through cable, the color order will be the same on both connectors. On a crossover cable,

the orange and blue pairs will be exchanged from one connector to the other.

Table 7-1.

UTP Ethernet cable wiring, straight-through

Wire color

Signal

1

Orange/White

Transmit (Tx) +

2

Orange

Transmit (Tx) -

3

Green/White

Receive (Rx) +

4

Blue

5

Blue/White

6

Green

Receive (Rx) -

7

Brown/White

8

Brown

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Reference Manual for the Model DG824M Wireless ADSL Modem Gateway

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Network and Routing Basics

Cable Quality

A twisted pair Ethernet network operating at 10 Mbits/second (10BASE-T) will often tolerate low

quality cables, but at 100 Mbits/second (10BASE-Tx) the cable must be rated as Category 5, or

"Cat 5", by the Electronic Industry Association (EIA). This rating will be printed on the cable

jacket. A Category 5 cable will meet specified requirements regarding loss and crosstalk. In

addition, there are restrictions on maximum cable length for both 10 and 100 Mbits/second

networks.

Internet Security and Firewalls

When your LAN connects to the Internet through a router, an opportunity is created for outsiders

to access or disrupt your network. A NAT router provides some protection because by the very

nature of the Network Address Translation (NAT) process, the network behind the NAT router is

shielded from access by outsiders on the Internet. However, there are methods by which a

determined hacker can possibly obtain information about your network or at the least can disrupt

your Internet access. A greater degree of protection is provided by a firewall router.

What is a Firewall?

A firewall is a device that protects one network from another, while allowing communication

between the two. A firewall incorporates the functions of the NAT router, while adding features for

dealing with a hacker intrusion or attack. Several known types of intrusion or attack can be

recognized when they occur. When an incident is detected, the firewall can log details of the

attempt, and can optionally send email to an administrator notifying them of the incident. Using

information from the log, the administrator can take action with the ISP of the hacker. In some

types of intrusions, the firewall can fend off the hacker by discarding all further packets from the

hacker’s IP address for a period of time.

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