Before you start
Objectives: learn the difference between network, subnet, internetwork, Internet, intranet, and extranet.
Prerequisites: you should know what do we need to create basic computer network.
Key terms: postal service analogy, cables, connectors, ip address, network, subnet, local area network, wide area network, internet, intranet, extranet
Postal Service Analogy
Cables and Connectors
Imagine that we have one road and on that road we have lots of houses. All houses have pathways that lead to the entrance of the house. In this case we can look at the road as being the network cable that connects each house together. There’s even a pathway that connects the house to the road, that is, connector that connects the computer to the cable. In the early days, networks used to look just like this. They had a long trunk cable with drop cables that were punched into the trunk cable that connected each of the devices together.
Now, each house on this street share the same street name, but each house has it’s own number. Notice how the address is actually a combination of the house number with the street address (street name). In networks we have similar situation. Each computer will have an address for the network followed by a host address. IP address contains the network address and a host address of a computer or device.
Networks and Subnets
We can have more streets that are connected with intersections. Different streets have different names, but houses on one street can have the same numbers as houses on other street. In networking terms we might say that one host is on network A and another host on network B. Different host can have the same host address if they are on different networks. In this case, a better word to describe a network would be a subnet. A subnet is a segment of the network that has the same subnet, or network address. In networking terms, all devices on one subnet share the same subnet address even though they have different host addresses. The device that connects two separate subnets within a network is called a router.
Local Area Network and Wide Area Network
To take this analogy a little further, we could imagine multiple streets that are located in one city area which is then connected with another city area with multiple streets. In this example, each street would be a subnet within a network, and the collection of networks in a close proximity would be a local area network. We can also have one city connected with another city with an interstate. In this case, each city would be a local area network and the connection between the two is a WAN link, or a Wide Area Network link. In this case the collection of the two cities, or the two LANs, connected together becomes the WAN, or the wide area network.
Network Under One Organization
Another way that we use the term network is to describe a network under the control of one organization. For example, there are two organizations and each one has it’s own network. In this case those networks are separated. If we connect those two networks with a WAN link, we can call it Wide Area Network because we have a WAN link between them. But, we might not call it a single network. Instead, we might use the term Internetwork. In this case we are talking about two separate networks which are connected together. A network is often a term that we use to describe a local area network or even a wide area network under the control of a single organization. An internetwork is often the term that we use to describe multiple networks that are connected together and that are managed by different organizations.
Now, we are close to revealing what an Internet is. Internet is an interconnection of networks. It is a collection of networks that all communicate together. Different parts of the Internet are typically not under a single administrative entity. To access the Internet we have to go trough the access point which is managed by the Internet Service Provider or ISP. The ISP is responsible for allowing individual users or companies with large networks to connect to the Internet. If we connect to the Internet, we are actually connecting our home network or company network to the Internet. Typically, there will be a security boundary that will separate our private network from the public Internet. Users and devices from our network can access the Internet, but computers that are on the Internet are not allowed to contact devices on our private network. Often, within our private network, we want to make our servers available on the public network. In that case, we would place our servers outside of the security boundary so that users from the Internet can access, for example, our web site that is located on our server. In that way we have made a small portion of our private network publicly accessible.
Another term that we have to be familiar with is an Intranet. An Intranet is a private network that uses Internet technologies, and it may or may not be connected to the public Internet. On intranet, services like e-mail server or web server are only available to users on our private network, even if our network is connected to the Internet.
Another term that we should mention is extranet. An extranet is basically an intranet that is made partially available, or partially public. For example, we could make a small portion of our network, that is connected to the Internet and is accessible only by particular users or group of users. So, extranet is made available to external users but only to the authorized external users, not to the general public.
When talking about the Postal Service analogy, house is computer, road is the network cable, pathway is the connector, street name is the network part of IP address, house number is the host part of IP address, different streets are different subnets, multiple connected streets in a city are like multiple connected subnets which form a Local Area Network, multiple connected cities are alike multiple connected Local Area Networks which form a Wide Area Network. The device that connects two separate subnets within a network is called a router. All devices on one subnet share the same subnet address even though they have different host addresses. Different host can have the same host address if they are on different subnets. The term Internetwork means that we have multiple networks connected that are managed by different organizations. The term network means that have a single network, or multiple networks connected that are all managed by single organization. Internet is an interconnection of networks, or a collection of networks that all communicate together. An Intranet is a private network that uses Internet technologies, and it may or may not be connected to the public Internet. An extranet is basically an intranet that is made partially public, but only to the authorized external users.