Question : IP address

I work in a company where we have a small network of computers. Please clarify the relationship between my network IP address and the IP address assigned by the ISP company.

My guess is that my network IP address always stays the same while the IP address assigned by the ISP company changes each time I connect. If so, why is this?

Thanks.

Answer : IP address

The IP address assigned by the ISP company is a real internet connected IP address.  Most commonly for home users, this address is dynamic, this means it issued from a DHCP (dynamic host configuration protocal) server at the ISP and may change from time to time (anything between once every 4 hours, or once a week or more).  Some ISPs issue a static IP address (ie one that is always the same), this service usually costs more, as real internet IP addresses are limited and need to be purchased by the ISP.  

There are ranges of IP addresses that are set aside as private IP addresses.  These addresses are not directly routable from the internet.  Most businesses and small home networks use these addresses as they do not need to be purchased.  Examples are (x can be any number between 0-255, but usually 1-254)

10.x.x.x , 192.168.x.x, 172.x.x.x

The 169.254.x.x (I think) is an autoconfiguration range puchased by Microsoft  - These are like private addresses and cannot be internet host addresses.

At your business, they probably use a private address range, but whether they are static (always the same), or dynamic depends on how the network is setup.  If there are a lot of computers and a server, there is usually a DHCP server, and most internal addresses are dynamically assigned.  If there are only a few computers, and no server, static is more common.  If internet connection sharing with windows 98 or win2000 or Winxp is used, It makes the computer with the internet connection into a mini, limited DHCP server, using the network 192.168.0.x,  and local addresses are usually dynamically assigned.  

On a windows NT or 2000 or xP (and sometimes 98 and ME) you can tell if your computer is using DHCP to get its address with IPconfig.  In a command box type ipconfig /all

 STart|RUN|COMMand  Enter

See the line DHCP Enabled- in this example it says no, and the IP address is statically assigned.

Ethernet adapter Local Area Connection DE500:

        Connection-specific DNS Suffix  . : corp.xxxxx.com
        Description . . . . . . . . . . . : Intel 21140 Based
Adapter
        Physical Address. . . . . . . . . : 00-00-F8-02-E3-E8
        DHCP Enabled. . . . . . . . . . . : No
        IP Address. . . . . . . . . . . . : 192.168.1.2
        Subnet Mask . . . . . . . . . . . : 255.255.254.0
        Default Gateway . . . . . . . . . : 192.168.1.1
        DNS Servers . . . . . . . . . . . : 192.168.1.2
                                            61.9.208.15

As larstr mentioned most businesses and home networks tha shared an internet connection between several PCs use NAT (Network address translation), which allows all the business computers to use the single internet IP address assigned by the ISP to talk to and from the internet.  This is usually done through a router, or special software.  (the internet connection sharing that comes with windows is a limited version of this type of software).

Regards

Anne