Reference Manual for the Broadband Voice Adapter TA612V

Network and Routing Basics

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Subnet addressing allows us to split one IP network address into smaller multiple physical

networks known as subnetworks. Some of the node numbers are used as a subnet number instead.

A Class B address gives us 16 bits of node numbers translating to 64,000 nodes. Most

organizations do not use 64,000 nodes, so there are free bits that can be reassigned. Subnet

addressing makes use of those bits that are free, as shown below.

Figure B-2:

Example of Subnetting a Class B Address

A Class B address can be effectively translated into multiple Class C addresses. For example, the

IP address of 172.16.0.0 is assigned, but node addresses are limited to 255 maximum, allowing

eight extra bits to use as a subnet address. The IP address of 172.16.97.235 would be interpreted as

IP network address 172.16, subnet number 97, and node number 235. In addition to extending

the number of addresses available, subnet addressing provides other benefits. Subnet addressing

allows a network manager to construct an address scheme for the network by using different

subnets for other geographical locations in the network or for other departments in the

organization.

Although the preceding example uses the entire third octet for a subnet address, note that you are

not restricted to octet boundaries in subnetting. To create more network numbers, you need only

shift some bits from the host address to the network address. For instance, to partition a Class C

network number (192.68.135.0) into two, you shift one bit from the host address to the network

address. The new netmask (or subnet mask) is 255.255.255.128. The first subnet has network

number 192.68.135.0 with hosts 192.68.135.1 to 129.68.135.126, and the second subnet has

network number 192.68.135.128 with hosts 192.68.135.129 to 192.68.135.254.

Note:

The number 192.68.135.127 is not assigned because it is the broadcast address

of the first subnet. The number 192.68.135.128 is not assigned because it is the network

address of the second subnet.

7262

Class B

Network

Subnet

Node

Reference Manual for the Broadband Voice Adapter TA612V

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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

masklength formats.

Configure all hosts on a LAN segment to use the same netmask for the following reasons:

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

Reference Manual for the Broadband Voice Adapter TA612V

Network and Routing Basics

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202-10037-01, March 2005

•

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

Choose your private network number from this range. The DHCP server of the TA612V adapter 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 computers 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

TA612V adapter employs an address-sharing method called Network Address Translation (NAT).

This method allows several networked computers 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 Broadband Voice Adapter TA612V

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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 computer (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 Broadband Voice Adapter TA612V

Network and Routing 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 computer accesses a resource by its descriptive name, it first contacts a DNS server to

obtain the IP address of the resource. The computer 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.