Reference Manual for the Wireless Cable Modem Gateway CG814WG v2

Networks, Routing, and Firewall Basics

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Appendix B

Networks, Routing, and Firewall Basics

This chapter provides an overview of IP networks, routing, and firewalls.

Related Publications

As you read this document, you may be directed to various RFC documents for further

information. An RFC is a Request For Comment (RFC) published by the Internet Engineering

Task Force (IETF), an open organization that defines the architecture and operation of the Internet.

The RFC documents outline and define the standard protocols and procedures for the Internet. The

documents are listed on the World Wide Web at

www.ietf.org

and are mirrored and indexed at

many other sites worldwide.

Basic Router Concepts

Large amounts of bandwidth can be provided easily and relatively inexpensively in a local area

network (LAN). However, providing high bandwidth between a local network and the Internet can

be very expensive. Because of this expense, Internet access is usually provided by a slower-speed

wide-area network (WAN) link such as a cable or DSL modem. In order to make the best use of the

slower WAN link, a mechanism must be in place for selecting and transmitting only the data traffic

meant for the Internet. The function of selecting and forwarding this data is performed by a router.

What is a Router?

A router is a device that forwards traffic between networks based on network layer information in

the data and on routing tables maintained by the router. In these routing tables, a router builds up a

logical picture of the overall network by gathering and exchanging information with other routers

in the network. Using this information, the router chooses the best path for forwarding network

traffic.

Routers vary in performance and scale, number of routing protocols supported, and types of

physical WAN connection they support. The CG814WG v2 Wireless Cable Modem Gateway is a

small office router that routes the IP protocol over a single-user broadband connection.

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Networks, Routing, and Firewall Basics

Routing Information Protocol

One of the protocols used by a router to build and maintain a picture of the network is the Routing

Information Protocol (RIP). Using RIP, routers periodically update one another and check for

changes to add to the routing table.

The CG814WG v2 Gateway supports both the older RIP-1 and the newer RIP-2 protocols. Among

other improvements, RIP-2 supports subnet and multicast protocols. RIP is not required for most

home applications.

IP Addresses and the Internet

Because TCP/IP networks are interconnected across the world, every machine on the Internet must

have a unique address to make sure that transmitted data reaches the correct destination. Blocks of

addresses are assigned to organizations by the Internet Assigned Numbers Authority (IANA).

Individual users and small organizations may obtain their addresses either from the IANA or from

an Internet service provider (ISP). You can contact IANA at www.iana.org.

The Internet Protocol (IP) uses a 32-bit address structure. The address is usually written in dot

notation (also called dotted-decimal notation), in which each group of eight bits is written in

decimal form, separated by decimal points.

For example, the following binary address:

11000011

00100010

00001100

00000111

is normally written as:

195.34.12.7

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.

Reference Manual for the Wireless Cable Modem Gateway CG814WG v2

Networks, Routing, and Firewall Basics

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Figure B-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 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.

7261

Class A

Network

Node

Class B

Class C

Network

Node

Network

Node

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Networks, Routing, and Firewall Basics

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

11111111

11111111

00000000 (255.255.255.0)

Equals:

11000000

10101000

10101010

00000000 (192.168.170.0)

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.

Reference Manual for the Wireless Cable Modem Gateway CG814WG v2

Networks, Routing, and Firewall 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