NETGEAR DG824M Router Manual PDF (Setup & Configuration Guide)

Page 111 / 154 Scroll up to view Page 106 - 110

Reference Manual for the Model DG824M Wireless ADSL Modem Gateway

Network and Routing Basics

B-3

The latter version is easier to remember and easier to enter into your computer.

In addition, the 32 bits of the address are subdivided into two parts. The first part of the address

identifies the network, and the second part identifies the host node or station on the network. The

dividing point may vary depending on the address range and the application.

There are five standard classes of IP addresses. These address classes have different ways of

determining the network and host sections of the address, allowing for different numbers of hosts

on a network. Each address type begins with a unique bit pattern, which is used by the TCP/IP

software to identify the address class. After the address class has been determined, the software

can correctly identify the host section of the address. The follow figure shows the three main

address classes, including network and host sections of the address for each address type.

Figure 7-1: Three Main Address Classes

The five address classes are:

•

Class A

Class A addresses can have up to 16,777,214 hosts on a single network. They use an eight-bit

network number and a 24-bit node number. Class A addresses are in this range:

1.x.x.x to 126.x.x.x.

•

Class B

Class B addresses can have up to 65,354 hosts on a network. A Class B address uses a 16-bit

network number and a 16-bit node number. Class B addresses are in this range:

128.1.x.x to 191.254.x.x.

Class A

Network

Node

Class B

Class C

Network

Node

Network

Node

Page 112 / 154

Reference Manual for the Model DG824M Wireless ADSL Modem Gateway

B-4

Network and Routing Basics

•

Class C

Class C addresses can have 254 hosts on a network. Class C addresses use 24 bits for the

network address and eight bits for the node. They are in this range:

192.0.1.x to 223.255.254.x.

•

Class D

Class D addresses are used for multicasts (messages sent to many hosts). Class D addresses are

in this range:

224.0.0.0 to 239.255.255.255.

•

Class E

Class E addresses are for experimental use.

This addressing structure allows IP addresses to uniquely identify each physical network and each

node on each physical network.

For each unique value of the network portion of the address, the base address of the range (host

address of all zeros) is known as the network address and is not usually assigned to a host. Also,

the top address of the range (host address of all ones) is not assigned, but is used as the broadcast

address for simultaneously sending a packet to all hosts with the same network address.

Netmask

In each of the address classes previously described, the size of the two parts (network address and

host address) is implied by the class. This partitioning scheme can also be expressed by a netmask

associated with the IP address. A netmask is a 32-bit quantity that, when logically combined (using

an AND operator) with an IP address, yields the network address. For instance, the netmasks for

Class A, B, and C addresses are 255.0.0.0, 255.255.0.0, and 255.255.255.0, respectively.

For example, the address 192.168.170.237 is a Class C IP address whose network portion is the

upper 24 bits. When combined (using an AND operator) with the Class C netmask, as shown here,

only the network portion of the address remains:

11000000

10101000

10101010

11101101 (192.168.170.237)

combined with:

11111111

00000000 (255.255.255.0)

Equals:

11000000

10101000

10101010

00000000 (192.168.170.0)

Page 113 / 154

Reference Manual for the Model DG824M Wireless ADSL Modem Gateway

Network and Routing Basics

B-5

As a shorter alternative to dotted-decimal notation, the netmask may also be expressed in terms of

the number of ones from the left. This number is appended to the IP address, following a backward

slash ( / ), as “/n.” In the example, the address could be written as 192.168.170.237/24, indicating

that the netmask is 24 ones followed by 8 zeros.

Subnet Addressing

By looking at the addressing structures, you can see that even with a Class C address, there are a

large number of hosts per network. Such a structure is an inefficient use of addresses if each end of

a routed link requires a different network number. It is unlikely that the smaller office LANs would

have that many devices. You can resolve this problem by using a technique known as subnet

addressing.

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 7-2: 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.

7262

Class B

Network

Subnet

Node

Page 114 / 154

Reference Manual for the Model DG824M Wireless ADSL Modem Gateway

B-6

Network and Routing Basics

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.

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.

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.

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

Page 115 / 154

Reference Manual for the Model DG824M Wireless ADSL Modem Gateway

Network and Routing Basics

B-7

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 DG824M 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.

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

Table 7-2.

Netmask Formats

Rate

Popular NETGEAR Models

Famous Router IPs

Famous Router Companies

Bookmark Our Site

Share