IP Addresses for LANs

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IP Addresses for LANs

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There are currently two systems for IP addressing – IP version 4 and IP version 6.  Version 6 is important because the world is running out of IP version 4 addresses, as there are only a little more than 4.23 trillion addresses, and we have used most of them up. We are going to concern ourselves in this article with v4 because on a Local Area Network (LAN) there is a neat scheme called Network Address Translation (NAT) that lets us re-use some IPv4  addresses in different local networks, and because IPv4 is much simpler.

IP v4 addresses consist of four, (up to) three digit numbers separated by periods or “tacks” – nnn.nnn.nnn.nnn – where each nnn can be a number between 0 and 255 (that is as much as 8 bits can hold).  192.168.20.3 is an example of a IP v4 address, while 320.168.20.3 would not.  Since a network consists of a number of computers, printers, set top boxes, internet radios, etc. we need to define what IPv4 addresses (numbers) are in your piece of the network called a “subnet”.  This can be done by a construct called a netmask.

An example of a subnet designation would be 192.168.20.0\24 (8 bits x 3 =24 bits) or 192.168.20.0 netmask 255.255.255.0 (another way of saying 24 bits) which says that the last number in the address of a device can be anything from 1 to 254 (0 and 255 are for special purposes)and it is in the subnet.  This particular example is a “Class C” network with 254 addresses available for assignment.  A netmask of 255.255.255.254 would only include two addresses

There are several sets of network addresses that are set aside for LANs as they have been made “non-routable” and cannot be routed over the internet. Any of these can be used for LAN addressing behind a home or office router.

They consist of :

  • 192.168.0.0 – 192.168.255.255  – 255 contiguous Class C networks using 192.268.nnn.000 netmask 255.255.255.0
  • 10.0.0.0 – 10.255.255.255.255 – One Class A network using 10.0.0.0 netmask 255.0.0.0
  • 172.16.0.0 – 172.31.255.255 – 16 contiguous Class B network using 172.16.0.0 netmask 255.240.0.0

A small office LAN will normally be connected to a router that handles the administration of the LAN IP addresses and connects to the internet through a Network Address Translation where communication is done to the internet or your Wide Area Network (WAN) using the IP address(es) of the router, and the LAN computers “Masquerade” as the router address when communicating outside the LAN.

The router usually includes a service called Dynamic Host Configuration Protocol (DHCP), although some networks have this taken care of by a server computer on the network.  DHCP is insufficient for some network components like servers and printers because it is not a sure thing that a device will get the same address back each time it is turned on.  There are some technologies to get over this such as uPnP for printers and other devices.  There are two older techniques which work well but require manual intervention to set up.

Static addressing is the most straightforward.  The computer ethernet interface is setup to (a) specific address(es) within the subnet.  There are several limitations to this.  First, the IP  address must not be also assigned by the DHCP server on the network.  This can be accomplished by configuring the DHCP server so only a portion of the subnet addresses can be issued by the DHCP Server, and the static addresses are within the subnet address space, but not among those that the DHCP server will issue.  The second problem is that the addresses statically configured must be kept track of – manually, usually in a spreadsheet so they can be reached by other computers, and so you don’t assign the address to another statically configured computer.  Another consideration is that many routers keep track of the hostnames of the computers they assign DHCP addresses to.  Usually these office and home routers automatically enter this information into their Domain Name Servers (DNS) which translate www.google.com to an IP Address.  They do this for the computers on the LAN also.  The router has no knowledge of the statically assigned computers, and cannot provide this service for them.

A better solution is to have the DHCP server on the LAN to reserve certain IP addresses for the particular Media Access Connection (MAC) addresses of the ethernet interfaces on  computers and devices on the net that need to always have the same IP Address.  These addresses are printed on the devices, and can be displayed using the ifconfig command on Linux and ipconfig command on Windows. Most modern office and home routers have this function.  You usually have to dig through the DHCP connections list to find the menu item to do this.  The manufacturers think that this is a seriously advanced feature.  On some routers it can be tricky if the computer has already been issued an IP Address by the DHCP server.

If your router has the MAC reserved IP Address function (sometimes the menu calls it Static – Confusing isn’t it?) then it is generally the preferable method.  If your network has no DHCP server, then Static IP Addresses are the only route available.  The Music Networks described throughout this site generally are simple Static assigned networks, while the Office networks have DHCP assignments.


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