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IP Addresses

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by wesley.chin

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A couple of questions:

- If the IP of a computer is dynamic, will it change every time there is a reboot?

- If a computer and a laptop connect to the same wireless router or hub, will the IP addresses be different on each?

- Is there a way for a remote computer to have access to a home network through the IP of a computer connected to the home network?

8 replies

by dan.cox

A dynamic address may not change every time you reboot a computer.
That will depend on the DHCP configuration.
If DHCP is configured such that an address is good for say three days and you reboot after one day, then when your computer comes back up, it looks at it's DHCP lease and says that the address it has is still good for two days. No need to renew.

Yes, if you connect a laptop and a desktop to the same router they will get separate addresses. No two computers can use the same IP address. They must have separate IP Addresses.

Not sure exactly what you mean by the last question.
Sorry.

by thumbsup2

This sure sounds like a homework assignment, but I'll go ahead and give you some basic information.

- If the IP of a computer is dynamic, will it change every time there is a reboot?

If talking about a LAN, theoretically, yes. In the real world though, most routers remember the address assigned to a specific MAC address and give it the same IP each time it connects.

- If a computer and a laptop connect to the same wireless router or hub, will the IP addresses be different on each?

Yes, they MUST each have a different IP, or you have a collision (not good).

- Is there a way for a remote computer to have access to a home network through the IP of a computer connected to the home network?

In simple terms, yes. However, it's done through the router first which redirects the traffic to the specific internal IP address of a computer. Now, if there is only one computer connected to the internet and no router involved (which means you don't have a LAN), yes indeed that computer can be reached from outside. That's why we all put firewalls (hardware or software) in place.

by wesley.chin

Thanks for the reply.

No, this is not a HW assignment.

The computers here all access the Internet through a T1 line that is shared via a hub. It seems that ever since getting the T1 line, the IP addresses for each computer have remained the same since day one. Yet the ISP says the IP is dynamic.

My third question was relating to telecommuting. One computer is able to share files with other local computers. If a certain folder of that computer is made downloadable on the Internet via an IP adress, could someone gain access to the contents of the other local computers?

by tintoman

Then your Internet address changes most likely whenever your modem/router is restarted or the connection is dropped for any reason.
This has no effect at all on the IP addressing of your network computers which are always on a limited lease.
As regards sharing files on the internet, you would need to assign a hostname to the computer concerned and sign up for an account with someone like DynDNS (It's free I think) who will then track your ever changing WAN IP address and assign your chosen hostname to it, that way anyone who wants to download your file can find the computer hosting it by using the host name.
However DO NOT share your network files on the internet, if you want to do this place the files on an FTP server with lots of security measures

by wesley.chin

I have a telecommuter who works in another state. The telecommuter has a company provided laptop that connects to Internet by a home network. The telecommuter is concerned that someone from the office may access files on the home network through the company provided laptop. Is this possible?

by cg it

a routable address assigned by an ISP is really an address on their network. You can buy 2 types of addresses. Static and dynamic.

With either type of address, you can hook up a computer to the demarc point, assign it that ISP provided address and volia! your on their network. The down side is that only that computer can be on the network. ISPs typically register MAC addresses of hosts and only allow that MAC address to connect. So if you wanted to have other computers also use that connection, you would have to deploy a router to do one to many NAT AND clone the MAC address from the computer to the router.

A typical ISP dyanmic assigned address lasts about 6 to 12 months. This cuts down on DHCP traffic on the network which if you think about the thousands of customers an ISP has all sending out DHCP renew packets every couple days, makes sense.

Now on to your telecommuter's company provided laptop.

If the laptop is part of the company domain, and the home user's other computers are not, then no, a company employee can't access files on other computers on an employees home network. Couple of reasons. First the home users routers firewall will block certain traffic that could be used for exploits. Second, virtually all computers that run XP have SP2 which is a firewall on the computer. So there are 2 layers of protection. Third is computers not belonging to a company domain belong to a workgroup. this workgroup acts as a security boundry. only computers that belong to the same workgroup can access shared resources.

So no, an employee at work can't access a computer on a telecommuters home network that isn't part of the companies domain using the telecommuters connection from home to work.

Yes, employees at work could access resources on the laptop at home provided that 1. there are shared resources on the laptop that the company employee has permission to access. 2. The laptop must be on the company subnet for company employees to be able to access shared resource on the laptop. Typically this is done via DHCP relay agent and VPN connections.

by kjell_andorsen

It looks like your initial set of questions have been answered. I get the feeling you're needing some more information that may not come through clearly in your questions though. We may need a little more information to help with your situation, but here are some things I'd like to point out as well.

If I understand you correctly you have a small group of users, presumably set up in a workgroup configuration that all connect to a shared T1 line through a hub, is that correct?

This seems extremely odd to me, you usually don't connect a hub directly to a T1 line, in fact, hubs are very rarely used at all as they have been replaced by much more efficient switches. However in this case I would think that the T-1 is actually connected to a router.

Also in a configuration like this it would be common for the router to provide what's called NAT functionality. This means that the IP addresses assigned to individual workstations are part of what's called a Private Address range. There are 3 commonly used private address ranges commonly used, and they are by designed never used on the internet itself, only in private networks. The router does the job of translating the private addresses used on the LAN to a public IP address used on the internet.

Do the workstations all have IP addresses starting with 10.x.x.x , 172.16.x.x or 192.168.x.x ? If so you are using NAT functionality. This means that all the workstations will have unique IP addresses on the LAN, but when going through the router, it will appear that they all have the same IP address on the internet.

This can make connecting to your LAN a little tricky if you're trying to access it from a remote system unless you have a VPN system set up.

If you give us a more detailed description of the network and what exactly you're trying to do we might be able to offer more solutions.

It also seems as if you might need a more thorough grounding in how TCP/IP works. There are lots of good guides and books out there. If you pick up a study guide for a basic networking cert like Network+ or Cisco's new CCENT certification you should get at least a good grasp of the basics.

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