Understanding the basics

Polish up on your fundamental computer networking knowledge.
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This month's article will explore networking basics from the perspective of someone who is new to the subject. Next month, we will look at networking for professional video applications from the perspective of someone who has general networking experience, but who has not worked on networks in this industry before.

Defining TCP and IP

Transaction Control Protocol (TCP) and Internet Protocol (IP) are two core protocols of the Internet. They almost always work together, but they are actually two separate protocols. TCP/IP indicates that an application is sending network information to the TCP layer and that TCP then sends its packets to the IP layer. From there, the IP layer sends its packets to a physical medium, which is almost always Ethernet.

Network protocols are layered on top of each other. This allows a designer to substitute different networking components with similar functionality at a particular layer without having to rewrite the entire networking application. Layered network protocols are known as a protocol stack. (See Figure 1.)

Almost all business networking uses TCP/IP. Let's begin our discussion of TCP/IP by using an example network comprised of a small office containing five computers that are connected to the Internet using a router.

Assigning IP addresses

TCP/IP networks are built using a numbering system composed of groups of numbers separated by periods (e.g. The group of numbers is known as an IP address. Each device on a network must have a unique IP address. Another group of numbers associated with the IP address is called a subnet mask (e.g. The subnet mask is common to a grouping of computers and networking devices and tells individual workstations the number of possible computers on the local network.

Before building our network, we need to decide what IP addresses to use. The range of acceptable addresses for a local network has been determined by the Internet Corporation for Assigned Names and Numbers (ICANN), the governing body of the Internet. In the early days of the Internet, developers realized that they needed documents to describe how the Internet functioned. These documents, created by the Internet Engineering Task Force (IETF), are known as Request for Comments (RFC). The IETF also generated a handful of standards (STD). IETF STD 07 defines TCP.

RFC 1918, “Address Allocation for Private Internets,” defines IP addresses for private networks. (See Table 1.) It sets aside three blocks of IP addresses for private networks. The block of IP addresses we use depends on the number of network devices we plan to install.

For our example, I have selected the address range —, with a subnet mask of You could choose any valid group of addresses shown in Table 1. The subnet used for our local network is different from the subnet shown in Table 1. By applying the subnet mask, we are using a small number of the total number of Class A IP addresses set aside by RFC 1918. The IP address block can be further divided depending on the number of devices on the network. (See Table 2 on page 30.)

As you can see from the table, the number of addresses in a network can be altered by changing the subnet mask. Any subnet mask from to will work in our sample network.

The subnet mask in a small company network using private addresses is not that important; most use Subnet masks become much more important when configuring routers connected to the Internet and in larger corporate networks.

For our example network, I assigned our first IP address in the block,, to the router. (See Figure 2 on page 34.) A router connects two physical networks together. In our case, it will connect our local (private) network to the Internet. The router is nothing more than a computer with two network cards.

The port of the router that is connected to the local network is referred to as the local area network (LAN) port. The side of the router that is connected to the Internet is referred to as the wide area network (WAN) port. The LAN side of the router will use the IP address The IP address for the WAN side is obtained from and assigned by our Internet service provider (ISP). In this example, the ISP assigned our public IP address as

The private IP addresses defined in RFC 1918 are unroutable, which means they cannot be projected onto the Internet. If you want computers on the LAN side of the router to be able to access the Internet, you will need to use a translator, commonly known as Network Address Translation (NAT).

Almost all routers have NAT built into them, and in simple home and office routers, this functionality is configured automatically. The router's WAN address is a public IP address, which means that anyone on the Internet can access the router by typing in the IP address.

The NAT built into the router allows each workstation to access the Internet, but the actual IP address of the individual workstation is never projected onto the Internet. If you were looking from the Internet into our sample network, the activity of the individual workstation would appear as if it were the WAN address of the router (i.e. I have assigned the computers in our example network the IP addresses shown in Table 3.

Each of our network devices has a gateway address. This address must be present if the users are going to access anything outside of our local network. The gateway address tells a workstation to send all network traffic not bound for our local network to the router.

For example, if you are at a workstation and attempt to go to www.cisco.com, the computer first resolves the IP address for www.cisco.com to 198.133,.219.25. It sees that this address is not on the local network and forwards it to the gateway address, which is our router. The router looks at the address, sees that it is not on the LAN side of the router and forwards the packet on to its gateway, which is shown in the table as This process continues until the packet reaches

This can be illustrated by going to a computer, opening a system window and typing “tracert www.cisco.com” (without the quotes) and pressing “Enter.” (See Figure 3.) Each of the 15 hops represents a router, and the subsequent hop represents the gateway address of the previous router. The number of hops will vary depending on the route from your computer to Cisco.

Brad Gilmer is president of Gilmer & Associates, executive director of the Video Services Forum and executive director of the Advanced Media Workflow Association.

Send questions and comments to: brad.gilmer@penton.com

Table 1. Private network IP addresses are defined in RFC 1918 by the Internet Engineering Task Force. Block IP addresses Subnet mask Number of addresses Class A block — 16,777,214 Class B block — 1,048,574 Class C block — 65,534 Table 2. Depending on the number of devices on the network, an IP address block can be further divided. IP addresses Subnet mask Number of IP addresses — 254 10.1918.0 - 126 — 62 — Table 30 — 14 — 6 — 2 Table 3. The above IP addresses were assigned to our example network. Computer name IP addresses Subnet mask Gateway addresses Router LAN —
WAN — WAN — Workstation 1 Workstation 2 Workstation 3 Workstation 4 Workstation 5