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Content:: Internet Protocol Addresses

Networking - Understanding IP Addresses

A computer that connects to the "internet" requires an Internet Protocol, or IP, Address. An IP Address is like a telephone number for your computer, if you would like to communicate with another computer you will need an IP Address similar to a telephone system in which you need a telephone number to call another telephone.

An IP Address Consists of 4 block of numbers ranging from 1 - 255 with each block seperated by a period. An IP Address will appear look like "". With IPV4 there are currently 4,294,967,296 IP addresses available, some of which are reserved for Internal/Private networks, used for subnet broadcasts, and subnet routers. Currently we use IPV4 which is based on 32 bits of data, sometime in the future users will know an IP Address in IPV6 format which increases the 32 bits of data to 128 bits. Future IP addresses will appear in 8 blocks and have hexidecimal notations instead of decimal notations per addressing block. An example of an IPV6 IP Address would be "2001:0db8:85a3:08d3:1319:8a2e:0370:7334"

IP Addresses are given to you by your Internet Service Provider, or ISP. Each computer must have its own unique IP Address most ISP's only give you one IP Address.

Table of Contents Name Address Translation

Even though one IP Address is only good for one computer to connect to the internet you can set up a home network, with virutally an ulimited amount of computers, to filter all of its traffic through one IP Address. This IP Address filtering is known as "NAT'ing" along with IP Masquerading. NAT stands for Network Address Translation and will modify packet headers on the fly while IP Masquerading allows your Private Local Area Network, or LAN, to funnel its data through one Public IP Address.

It is very common to have a network applicance sometimes called "NAT'ing Routers", "Home Routers", or "Cable/DSL Routers" to handle the Network Address Translation and IP Masquerading for you. In most cases this internet magic is seamless to the end user.
You can also use a Linux system with the IP Tables firewall to act as your IP Masquerade with a few simple commands. # iptables -A FORWARD -i eth0 -m state --state NEW,INVALID -j REJECT
# iptables -t nat -A POSTROUTING -o eth0 -s $trusted_int -j MASQUERADE
# echo 1 > /proc/sys/net/ipv4/ip_forward
Remember to replace eth0 with whatever internet interface you are using to connect to the internet with.
An example shell script pertaining to IP Tables can be found here. Common types of NAT'ing router brands are Linksys, Dlink, Netgear, Netopia, etc. You can use the google shopping results for nat routers here

This effect of NAT'ing and IP Masquerading is very similar to a military chain of command. Where all of the soldiers complain to their sergeant, the sergeant complains to his superior so on and so forth. However in the case of the Internet the Soliders are the computers on the Private LAN and send all their communications through a "NAT'ing Router". One problem we run into with this structure is some communications on the internet must be done computer to computer via an internet port.

A Port is just as it sounds, something where ships would dock. On the internet a port allows communication between to occure between computers. There are different types of ports the type that acts like a shipyard dock would be a "listening" port. A listening port is a computer port that is always waiting to get information from other computers on the internet. Other ports are created (which are more like ships) that make communication to a listening port and stay open while the computers chatter, it is much like a shipping channel.
Internal vs External IP Addresses or Reserved IP addresses

An Internal IP address is an IP address that cannot be used to communicate on the internet and will only be good for a Private Network. The following are Internal or Private IP addressing Structures : or - or - or to
Static Versus Dynamic IP Addresses

By default most systems search for an IP Address obtainable through a DHCP server. When your system connects to a DHCP server you will receive a temporary IP Address for your session, this is called a Dynamic IP Address as it has potential to change on a whim. Your computer acting as a client to a network can have a Static or unchanging IP Address entered into your configurations. It is common for ISP's to charge an extra monthly fee for a static IP Address.
It is common for basic computer users to have a Dynamic IP address. Static IP Addresses are used for more of a server grade system. A website is an example of something that, in most cases, would require a static IP address.
DHCP Network default IP Address

If you are having trouble connecting to the internet and you receive an IP that starts with 169.254 or is part of the network structure. This can usually be a result of DHCP not being able to find a valid IP address. You can attempt to fix the problem on a Windows System by typing in # ipconfig /release followed by # ipconfig /renew This will remove all the information in your network configuration and attempt to refresh the DHCP connection. If the network you are connecting to does not have a DHCP server you may have to enter a static IP Address into your system.
Determine your IP Address

On a Microsoft Windows System you can type in # ipconfig Your output will look similar to the following
Windows IP Configuration

Ethernet adapter Local Area Connection:

        Connection-specific DNS Suffix  . :
        IP Address. . . . . . . . . . . . :
        Subnet Mask . . . . . . . . . . . :
        Default Gateway . . . . . . . . . :
On a Linux Style system (like MacOS) you can type in # ifconfig Your output will look similar to the following
eth0      Link encap:Ethernet  HWaddr XX:XX:XX:XX:XX:XX
          inet addr:  Bcast:  Mask:
          inet6 addr: XXXX::XXX:XXXX:XXXX:XXXX/XX Scope:Link
          RX packets:32555020 errors:0 dropped:0 overruns:0 frame:0
          TX packets:27261621 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:3611212648 (3.3 GiB)  TX bytes:690325935 (658.3 MiB)
          Interrupt:201 Base address:0xc000

lo        Link encap:Local Loopback
          inet addr:  Mask:
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:218 errors:0 dropped:0 overruns:0 frame:0
          TX packets:218 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:21192 (20.6 KiB)  TX bytes:21192 (20.6 KiB)

Last Modified: 2012-10-21