A Crash Course in TCP

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    A crash course in TCP/IP

    At first glance, TCP/IP (Transmission Control Protocol/Internet Protocol) may seem baffling. Many other protocols, suchas NetBEUI and IPX/SPX, require no configuration. TCP/IP is different. Due to the seemingly endless number of optionsthat you can configure within TCP/IP, many people become intimidated at first. In reality, however, TCP/IP isnt verydifficult, but you have to gain some understanding of what youre configuring. So, Im going to give you a crash course inTCP/IP. Although I wont be able to explore every feature in detail, Ill cover the important points.

    The IP address

    The most basic element of TCP/IP is the IP address. The IP address is a number thats unique to each computer. If youknow a computers IP address, you can communicate with that computer from anywhere in the world. Since TCP/IP is theprotocol that the Internet uses and since Internet servers are located all over the world, TCP/IP must be routable. Thus,when you try to access an IP address, your computer must be able to tell whether or not that IP address is located onyour local network. If the desired address is located on your local network, you wont have a problem reaching it. If it isnton your local network, TCP/IP must know which network the IP address is located on in order to reach the address.The network number represents the network that contains a given IP address. If you look through the various tabs of theTCP/IP properties sheet, youll see that theres no field that allows you to specify the network number. Instead, thenetwork number is part of the IP address.An IP address is composed of a network number and a computer number. Your computer can distinguish those twonumbers because of something called the subnet mask. The subnet mask is located in a field directly below the IPaddress on the TCP/IP properties sheet. A simple subnet mask would be something like 255.255.0.0. The numbers thatmake up the subnet mask indicate which portion of the IP address is the network number and which portion is thecomputer number. The four numbers of the subnet mask correspond directly to the four numbers in the IP address. For

    example, if you had a computer with an IP address of 147.100.100.25 and a subnet mask of 255.255.0.0, the first twonumbers in the subnet mask (both are 255) indicate that the first two numbers of the IP address are the network number.The second two numbers (both are 0) indicate that the second two numbers of the IP address are the computer number.Therefore, in the IP address 147.100.100.25, the 147.100 portion denotes which network the computer is located on, andthe 100.25 portion represents a particular computer on that network. Of course, subnet masks become much morecomplex than this example. For instance, you can subdivide an individual network.

    The default gateway

    Now that you know how TCP/IP determines whether a destination address is located on a local network or on a foreignnetwork, you may wonder how it attaches to a foreign network, especially if that network is halfway around the world.Well, it completes this job by using routing tables. Routing tables, which are stored in your routers, tell the router where toconnect in certain situations.You should note that one of the TCP/IP configuration options is for a default gateway. The default gateway is the addresswhere a TCP/IP packet is sent if TCP/IP cant locate the destination IP address on the local network. Usually, the defaultgateway points to the router that controls your buildings link to the outside world (often through an Internet connection).Since this router has a table of other routers, it knows the address and location of these routers. The destination IP

    address helps the router determine to which router to pass the packet. Typically, a TCP/IP packet is passed throughseveral routers before it arrives on the destination network and, finally, on the destination PC.

    DHCP

    If all of this information is making your head spin, theres one TCP/IP feature that will make your life easier: Dynamic HostConfiguration Protocol (DHCP). DHCP allows you to set one or more servers on your network to act as a DHCP server.When a client computer thats set to use the DHCP option comes online, the DHCP server automatically configuresTCP/IP on this computer. Of course, you still have to configure the DHCP server, but using DHCP saves you from all ofthe effort of configuring each client manually. It also keeps you from accidentally using a particular IP address more thanonce. DHCP is also useful if you have a limited number of IP addresses to work with; only the computers that are turnedon at a given moment will use IP addresses.

    WINS

    On Windows-based networks, each computer has a name. The WINS option can be configured to contain the IP addressof a Windows NT Server thats running a WINS service. The WINS service resolves computer names (NetBIOS names) toIP addresses. Thus, if you try to access a computer by its name, your computer will go directly to the WINS database andsearch for the name and IP address. It wont have to query every computer on the network to determine the right IPaddress. As you can imagine, using WINS really cuts down on network traffic.

    DNSDNS functions similarly to WINS, except that it manages domain names rather than computer names. A domain name isthe type of name that you use on the Internet. For example, Microsoft.com is a domain name. When you point yourbrowser towww.microsoft.com, your computer will query a DNS server for the IP address that corresponds to the domainname. If the DNS server doesnt know the address, it asks another DNS server. This process continues until a DNSserver somewhere along the line knows the address. Then, that server informs your computerand all of the other DNSservers that are involvedof the domain names IP address.

    http://www.microsoft.com/http://www.microsoft.com/http://www.microsoft.com/