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Computer Networks Part II

Several responses to the last tutorial pointed out how intricate and complex the world of IP networking is. From these responses, I felt that more coverage of the basics of IP networks is necessary.

Network overview

The data that is transported over an IP network is contained within an IP packet; this packet is the basic building block of every IP network. Designed and built to transport data over these networks, it contains both the source and destination addresses needed to move data to its final destination. Some IP packets will have to pass through multiple individual networks that have various requirements. To travel over these, the IP packet will have to adapt to the requirements of each one by being encapsulated within data frames that carry the required addressing and formatting needed by each different network. When the IP packet moves between different networks, these encapsulating frames will be discarded and new ones formed for the next network and its protocols. The addresses that the data frame carries are used to move the IP packet from one device to the next, and from one network to the next, as it moves toward its final destination. As the IP packet advances through these various networks, its original addresses and data are left intact.

To accomplish all of this, requests and responses are sent out over the network and/or the Internet to obtain the addresses required to direct the IP packet. Other devices on the network monitor these exchanges and keep tabs on the addresses transmitted, while others exchange information on paths that can be used to route IP packets to particular destinations.

All of this happens to the billions and billions of IP packets sent out over various networks every day, and it works because a set of standards called protocols have been developed, maintained and expanded over the years to keep the networks we depend on working. If you understand these protocols and how they work together, then you will be able to build, maintain and troubleshoot these IP networks. These tutorials will give you a glimpse into how IP networks move your data from point A to point B reliably and without errors.


The aforementioned network rules and requirements are called protocols, which, more specifically, are rules and specifications that define how data is organized, formatted and addressed as well as the electrical, wireless or even optical characteristics of the actual medium that carries the data, including the cable and connectors used.

Some protocols still exist from the inception of networks, and others are just now being deployed. However old they are, protocols allow equipment from different manufacturers to work together. In the past, almost all protocols were proprietary, and you could only buy compatible equipment from one manufacturer.

OSI model

The Open Systems Interconnections (OSI) reference model describes how different levels of software and hardware should interact with one another to send and receive data over a computer network. In other words, OSI is a framework for how a network protocol should be built. The actual OSI model never caught on as a standard, but it now acts as a reference model allowing different network protocols to be discussed and their functions compared. There are seven layers to the OSI model:

Layer 7 — Application
Layer 6 — Presentation
Layer 5 — Session
Layer 4 — Transport
Layer 3 — Network
Layer 2 — Data Link
Layer 1 — Physical

Most network protocols do not fit precisely into each of the OSI seven layers; functions sometimes overlap with others, but it still provides a useful framework for discussion.

Layer 1 is concerned with the electrical, wireless or optical characteristics of the actual medium that carries the data as well as the connectors and cables. Layer 2 is about the headers and trailers and when data can be sent (i.e. placed onto the wires) and some simple error checking. Layer 3 concerns addressing and the path the data will take through a network. Layer 4 is about getting the data to another computer, error recovery and flow control.

The higher layers cover the starting and stopping of the data exchange, how the data is encoded, receiving the data to be sent and delivering the received data to the computer program involved (e.g. Internet browser, FTP, e-mail).


IP stands for Internet Protocol and is the basis for most computer networking today. IP encompasses both addressing and the basic unit of data delivery. Most engineers are familiar with IP addresses (e.g., which are assigned to equipment that communicate over an IP network and must be unique on that network. Private networks are ones that are not directly accessible via the Internet. There are three IP address ranges designated for use on private networks, which avoid any confusion with actual Internet or public IP addresses. The ending addresses of and are reserved and not assigned to any equipment. The .255 address is used to broadcast requests over the LAN.

The IP packet is what actually transports the data over an IP network and is constructed by the software that approximates the layers mentioned above. As the data passes from one program or layer, new parts are added until the packet is complete. These IP packets are always encapsulated within data frames to pass through a particular network conforming to its protocols.


Subnets are used to divide up a network and decrease the amount of traffic on each one. Basically, a subnet is a subnetwork with its own unique range of IP addresses. Routers or gateways are equipment used to separate these subnetworks; no traffic on one network passes to the other unless certain requirements are met.

For example, a router is required to separate a company’s local area network (LAN) from the Internet. Traffic on the company LAN moves about without ever crossing the router. When someone needs to access the Internet by requesting data from an IP address that resides outside of the LAN’s IP address range, the Router is accessed and that data is passed through. The returning data from the Internet also passes through the router and back to the requesting computer.

A computer knows an IP address is outside of its range by use of a subnet mask. This mask filters out local traffic and only passes IP address that are not local. Each computer is supplied with the subnet mask for its network. If an IP packet is being sent to an address outside of its subnet mask, the packet is sent to the local router.

Routers strip off the incoming frame around the IP packet and add a new one for the network the IP packet is about to be sent to. In this way, the IP packet is always correctly formatted for the network it is traveling over.

Next Time

The next tutorial continues with computer network basics, moving to TCP/IP, MAC addresses and network switches.