Reference Manual for the Wireless Cable Modem Gateway CG814WG v2

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Networks, Routing, and Firewall Basics

The following table lists the additional subnet mask bits in dotted-decimal notation. To use the

table, write down the original class netmask and replace the 0 value octets with the dotted-decimal

value of the additional subnet bits. For example, to partition your Class C network with subnet

mask 255.255.255.0 into 16 subnets (4 bits), the new subnet mask becomes 255.255.255.240.

The following table displays several common netmask values in both the dotted-decimal and the

mask length formats.

Table 7-1.

Netmask Notation Translation Table for One Octet

Number of Bits

Dotted-Decimal Value

1

128

2

192

3

224

4

240

5

248

6

252

7

254

8

255

Table 7-2.

Netmask Formats

Dotted-Decimal

Masklength

255.0.0.0

/8

255.255.0.0

/16

255.255.255.0

/24

255.255.255.128

/25

255.255.255.192

/26

255.255.255.224

/27

255.255.255.240

/28

255.255.255.248

/29

255.255.255.252

/30

255.255.255.254

/31

255.255.255.255

/32

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Networks, Routing, and Firewall Basics

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NETGEAR strongly recommends that you configure all hosts on a LAN segment to use the same

netmask for the following reasons:

•

So that hosts recognize local IP broadcast packets

When a device broadcasts to its segment neighbors, it uses a destination address of the local

network address with all ones for the host address. In order for this scheme to work, all devices

on the segment must agree on which bits comprise the host address.

•

So that a local router or bridge recognizes which addresses are local and which are remote

Private IP Addresses

If your local network is isolated from the Internet (for example, when using NAT), you can assign

any IP addresses to the hosts without problems. However, the IANA has reserved the following

three blocks of IP addresses specifically for private networks:

10.0.0.0 - 10.255.255.255

172.16.0.0 - 172.31.255.255

192.168.0.0 - 192.168.255.255

NETGEAR recommends that you choose your private network number from this range. The

DHCP server of the CG814WG v2 Gateway is preconfigured to automatically assign private

addresses.

Regardless of your particular situation, do not create an arbitrary IP address; always follow the

guidelines explained here. For more information about address assignment, refer to RFC 1597,

Address Allocation for Private Internets,

and RFC 1466,

Guidelines for Management of IP

Address Space

. The Internet Engineering Task Force (IETF) publishes RFCs on its Web site at

www.ietf.org.

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

CG814WG v2 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)

.

Reference Manual for the Wireless Cable Modem Gateway CG814WG v2

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Networks, Routing, and Firewall Basics

The following figure illustrates a single IP address operation.

Figure B-3:

Single IP Address Operation Using NAT

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.

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

Reference Manual for the Wireless Cable Modem Gateway CG814WG v2

Networks, Routing, and Firewall Basics

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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)

.

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.

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Networks, Routing, and Firewall Basics

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 CG814WG v2 Gateway has

the capacity to act as a DHCP server.

The CG814WG v2 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.

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.