TCP/IP Tutorial: IP Routing - A Magical Element of the Internet

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Although the assignment of static IP addresses is usually taken care of by the IS/IT department, understanding the concepts and basic structure can be of considerable value. An installation and maintenance technician can, with this understanding, play a much more active and contributing role in the turn-up of IP elements. A manager can, with this understanding, better position their department to survive the transition from legacy transport to IP-based solutions. The widespread use of protocols, such as SMTP for telemetry reporting and TCP/UDP for telemetry transport, are evidence of this transition in process. These protocols, as well as SMTP, POP3, FTP, TELNET and a host of others, all rely on IP addressing. This tutorial will discuss the three fundamental items involved in typical element addressing - the IP Address, the Subnet Mask, and the Default Gateway.

A manager can, with this understanding, better position their department to survive the transition from legacy transport to IP-based solutions.

IP Address

The IP Address is perhaps the easiest one to understand. In legacy multi-drop polling environments, each element on the leg had to have a unique ID that the polling master used to collect data. In a LAN/WAN environment, each element on a network segment must similarly have a unique ID. This unique ID is technically a combination of IP and MAC addresses, but since the equipment manufacturer generally assigns the MAC address, the IP address becomes the controllable variable. Though the assignment is generally handled, as we discussed above, by the IS/IT department, it is good to remember that IP communication difficulty can sometimes be a simple issue of mistyping the assigned IP address when setting up the element. Another somewhat less frequent occurrence is the assignment of an already-used address to a new element. The contention resulting from this mistake will usually impair communication with both elements just as it did in legacy networks where two units would respond to the same poll.

Subnet Mask

The Subnet Mask can be simplistically understood as a filter that prevents everyone else from seeing the traffic on your leg of the network. The actual filter is a special network element that sits on your leg and handles traffic to and from other network legs. A standard element uses its Subnet Mask to determine when a request needs to be handled by that special element and when it can simply be placed on the local leg. The technical side of this function involves a logical XOR of the element's IP Address with the destination IP Address. The result is ANDed with an uninterrupted sequence of bits (the Subnet Mask) to enable a decision. If no bits survive the logical shenanigans, the traffic will be placed on the local leg. Otherwise, the traffic will be sent to the special element, which is typically called the Default Gateway. Obviously, an accurate Subnet Mask is imperative for traffic generated by an element. It is equally important even when the element is just responding. Though the request may reach the element, the response can be utterly prevented by a single bit error in the Subnet Mask that prevents the element from correctly routing the traffic to the Default Gateway.

Default Gateway

The Default Gateway must then, by definition, have an address on your local network leg. It also has separate connections and unique addresses on other legs. The Default Gateway processes traffic-routing requests received from each leg, decides where the traffic needs to be sent, and places it on the appropriate destination leg, using logical functions similar to the ones mentioned previously. If it has no leg that is suitable for the addressing of the routed request, it typically has a Default Gateway of its own to which it forwards the request. The maintenance of gateways is no casual affair and is not generally involved in the deployment of an IP element.

The routing of IP traffic is one of the magic elements of the Internet and entire volumes have been printed in exhaustive detail regarding every little aspect of the IP addressing and routing mechanism. This article has presented what is hoped to be an easily understood model of three items commonly involved in deploying elements in an IP environment. The IP Address which 'uniquely' identifies an element, the Subnet Mask which 'filters' both autonomous and response element traffic and the Default Gateway which provides 'operator assistance' for non-local traffic. In our next article, we will deploy a NetGuardian as a DCPx responder to illustrate how the concepts we have discussed up to this point play into a typical installation.

-Bob Berry