9 Apr 1995

Planning TCP/IP

A more general discussion of TCP/IP elements and configuration can be found in Introduction to TCP/IP

You check into a hotel room and want to make a phone call. To call someone in another room, dial "7" and the room number. The phone call is routed through the switchboard. To dial a local call, dial "9" and the phone number. The call is routed through the switchboard and local phone company. To dial long distance, dial "8", the area code, and the phone number. The call goes through the switchboard, the local phone company, and a long distance carrier.

A desktop computer wants to send a message to a server. If both are on the same LAN, then the message is transmitted directly using Ethernet rules. If the server is not connected to the LAN, then the message will have to pass through some gateway device. Every PC using TCP/IP has to be configured with information that answers the questions: What is local? What is long distance? Who handles long distance?

Each machine on the Internet has an address in the form of a four byte IP number. The number has traditionally been written by converting each of the four bytes to an integer (0 to 255) and separating the four numbers with a period. The first two or three bytes designate a network belonging to an organization. The remaining number designate individual computers.

Yale's network are all numbers beginning with "130.132". The next number is internally assigned to different departmental LANs, and the last number is a computer within that department.

Ethernet software requires a small set of values. The example below has been customized for the Yale environment:

       IP Address     130.132.xxx.yyy 

In this block of information, the xxx is replaced by a number 2-254 that represents your department or building. The yyy part is replaced by a number that represents your individual machine. You are also assigned a host name in place of the "zzz" part. Certain departments will have a domain name where "cis" has been replaced with a different designation (cs.yale.edu for the Computer Science department, med.yale.edu for the Medical School, etc.).

Lets consider this configuration from the point of view of the network software. Suppose that the department (xxx) is 233 and the station (yyy) is 212. The IP address of this machine is then Any other address that begins 130.132.233.* will be on the same Ethernet and can be reached directly. Any other IP address in the world is reached by sending the data through the Ethernet to the gateway at

TCP/IP configuration does not depend on an enormous amount of information, but it is claimed that a large percentage of the US public cannot program a VCR. It is unrealistic, and may be unnecessary, to expect everyone who uses a PC in a large organization to be able to enter such cryptic data on demand. To address this problem, Microsoft has built an automatic configuration server into Windows NT 3.5. The Dynamic Host Configuration Protocol (DHCP) is a new, improved version of the old Unix "bootp" service. The DHCP server is configured with a range of IP addresses in each subnet that can be "leased" out to machines. Individual addresses can be reserved for server machines based on hardware-level Ethernet identifier for the card. Other client machines borrow an unused address from a general pool of addresses.

Setting up a DHCP server may require more initial work. However, it provides a nearly foolproof installation of the client machine. The initial work will more than pay for itself in reduced service calls when users on manually configured workstations discover that their connection does not work because they have entered the wrong gateway or name server address.

When TCP/IP is installed in a network-aware Microsoft operating system (Windows for Workgroups 3.11 with the "Wolverine" package of TCP/IP support, Windows NT 3.5 Workstation, or Windows 95-Chicago) the default presented during configuration is to use a DHCP server to fill in all the required information. If this option is not selected, then the remaining boxes in the dialog allow the user to manually enter an address, mask, and router.

Machines that dial in to the Internet over phone lines are fairly simple to configure. The TCP/IP software should quickly recognize that everything is located on the other end of the phone line. However, the TCP/IP architecture pretends that the phone line is a tiny LAN with two computers, one at each end. It therefore must internally go through the same kind of configuration with addresses, masks, and gateways. Sometimes this dummy configuration is done automatically, but sometimes data must be captured from the dialog that occurs when the computer dials in and logs on to the network. This will be discussed under the general topic of logon scripts.

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Copyright 1995 PC Lube and Tune -- Windows on the World -- H. Gilbert