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[#] Sat Feb 14 2009 10:58:02 EST from Dirk Stanley @ Uncensored

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Can someone tell me what a DHCP lease is, and how it affects me? (Or the price of beans in China?)

[#] Sat Feb 14 2009 13:17:32 EST from Harbard @ Uncensored

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Sure, write out a check for $300.00 and mail it to me.

 

Seriously though, when your computer logs on a network it contacts a DHCP server.  That server assigns your computer an address so any traffice on the network can find your computer.  Periodically, that address may be reassigned.  Hence, they call it a lease.  As an end user it should be vitually transparent.  If you are a sys-admin or a geek that messes around with your system it might matter.



[#] Sat Feb 14 2009 23:13:51 EST from IGnatius T Foobar @ Uncensored

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Right. After that "lease" expires, the address technically isn't yours anymore, which is why a properly functioning DHCP client will eventually "renew" it (usually when the lease period is half over).

Although, a good DHCP server will let you get away with not renewing your lease, because before it hands that address out to someone else it'll ping it first to make sure there isn't anyone using it.

The lease period is set by the server administrator. On a very dynamic network with lots of clients coming and going all the time, the lease period might be very short -- perhaps half an hour at a place like a coffee shop. On a home or office network where it's the same computers all the time, 24 hours or more is common.

[#] Sun Feb 15 2009 06:43:21 EST from fleeb @ Uncensored

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And, just to be thorough, if your computer is not set up to accept a DHCP license, it could be set up to statically allocate its IP address, meaning the IP address is always going to be the same, because you chose it yourself, and assigned it to the network interface.

Some DHCP servers will allow you to associate an IP address to a requesting MAC address (read: network interface... all network interfaces have a MAC address), creating the opportunity for a static DHCP address.  This allows you to ensure a particular computer's IP address is always going to be the same when the system boots up, yet the system requests its IP address (and other information, like DNS servers, gateway, and network mask) from DHCP.  This is very useful for situations where you might remove your computer from your own environment and take it somewhere else occasionally, but you still want a stable IP address at your own home.  Or if you just want to keep all your IP addresses in one place; managing IP addresses across several computers can drive you nuts if you have to visit each computer to track what is going on.



[#] Sun Feb 15 2009 10:08:56 EST from IGnatius T Foobar @ Uncensored

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And now for a discussion about dynamic addresses on IPv6!

[#] Sun Feb 15 2009 10:57:55 EST from Harbard @ Uncensored

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Here's another situation.  A computer with two or more NICs....I was told each NIC needs it's own ip address.  True?



[#] Sun Feb 15 2009 11:05:04 EST from fleeb @ Uncensored

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Yes, I believe that's true.

Otherwise, the network would not know to which MAC address to direct a packet.

Besides, it would do the system no good to have both NICs assigned to the same IP, even if it were possible.

If the two NICs are on isolated network segments (NIC A is nowhere connected to NIC B) I suspect you might be able to use the same IP address, but then your computer might be confused as to where to send a network packet for distribution.  It would need to be designed to act as a bridge, I think, under such a situation.  Perhaps someone who knows more about this could comment.



[#] Sun Feb 15 2009 13:47:48 EST from Harbard @ Uncensored

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But couldn't two different MAC addresses share the same IP address?  That shouldn't cause problems as long as it is the same machine using both.  That would allow one machine to use more bandwidth, say on a gigabit network.  I have my house wired for gigabit speeds, but only 2 computers on the network most of the time.  It would be convient if I could increase the bandwidth to a particular computer for special needs.

 

Of course I don't really NEED to do this, I'm just screwing around.



[#] Sun Feb 15 2009 16:38:38 EST from Ragnar Danneskjold @ Uncensored

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There are ways of doing link aggregation, or nic teaming, but it can cause strange problems.

[#] Sun Feb 15 2009 22:29:22 EST from IGnatius T Foobar @ Uncensored

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Right. Usually when you have more than one interface, it's because you're connecting to multiple networks (as would be the case if the computer were acting as a firewall, for example).

If you want to do link aggregation, always make sure that you have the right drivers for it, and that they're matched up with a link aggregation mode that is supported by your switch. Then, before implementation, decide not to bother with it anyway because there's very little chance that your computer is capable of enough throughput to fill two channels anyway.

Some people connect multiple interfaces to the same network with different IP addresses. These people are called "morons." You can assign multiple IP addresses to a single interface and accomplish the same thing without extra hardware and cables.

If you like to experiment, a worthwhile project is to learn how to connect a *single* interface to *multiple* networks, using 802.1q VLAN trunking. This is something worth learning how to do if you ever plan to deploy servers on big networks.
I've got one machine that has a single ethernet interface bound to 65 different networks. It's providing firewall and VPN services for dozens of different subscribers, each on their own private VLAN. Wild stuff.

[#] Tue Feb 17 2009 12:42:36 EST from Dirk Stanley @ Uncensored

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Okay, dumb question alert :

1. I remember talk about IPv6 about 5 years ago - Are we actually using IPv6 now?

2. Re : DHCP lease - So does this mean my cable company can change my IP address on me? In my experinece, it's always the same - Why do I care then about my DHCP lease? What if I wanted to set my IP address to the same thing my neighbor uses?

[#] Tue Feb 17 2009 13:04:23 EST from dothebart @ Uncensored

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some sites use it meanwhile. I'm having trouble subscribing a sf.net mailinglist, because of postfix won't accept connections from ipv6 hosts if its running in ipv4 mode...



[#] Tue Feb 17 2009 13:11:38 EST from fleeb @ Uncensored

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Some places uses IPV6 (e.g. Chinese Olympics), and many places are still on IPV4.

IPv6 has infrastructure in place that negates the need for DHCP; DHCP is, therefore, a hack to work around IPv4 limitations.

[#] Tue Feb 17 2009 14:10:30 EST from Harbard @ Uncensored

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Tue Feb 17 2009 12:42:36 PM EST from Dirk Stanley <>
Okay, dumb question alert :

1. I remember talk about IPv6 about 5 years ago - Are we actually using IPv6 now?

2. Re : DHCP lease - So does this mean my cable company can change my IP address on me? In my experinece, it's always the same - Why do I care then about my DHCP lease? What if I wanted to set my IP address to the same thing my neighbor uses?

Yes, your cable company can change your ip address at anytime.  The ipdress is assigned to the MAC address of your cable modem.  So you can not be confused with your neighbor.  In fact you can several different ip addresses, but only one ip address per MAC address which is hardware based.  If you check, the ip address the cable company assigns to you is different from what you computer will report as it's ip address.

 

You computer uses an address that looks something like 192.168.1.101.  The outside world sees you computer as something like 76.95.30.16.  There are entire books written on the hierarchy of ip addresses.  IPv6 is supposed to simplify all of this.



[#] Tue Feb 17 2009 14:27:23 EST from Harbard @ Uncensored

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Nuts.  I left out part.  Your home router handles traffic between the cable modem and any computers on your home network.  For instance, in my home network the cable company has assigned me one IP address.  I have three computers and printer sharing that external Ip address though each one has it's own internal IP address.

 

I also find it amusing that the MAC address in router belongs to a NIC that hasn't existed in about 5 years.  Comcast want to charge you for each and every IP address you use.  Your router effectively isolates you network from the outside world so you can do whatever you want.

 

If you need a static external IP address there are work arounds.  For instance, I have my own Citadel running ( yggdrasill.servebbs.net  check it out) so I need a static IP or no one will be able to find it.  I use a service called DynDNS.com (it's free!).  I install a small program on my box that updates my IP address.  They then redirect any traffic to yggdrasill.servebbs.net to the correct address, regardless of what Comcast does.



[#] Tue Feb 17 2009 14:38:24 EST from Peter Pulse @ Uncensored

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Yes, your cable company can change your ip address at anytime. The ipdress

is assigned to the MAC address of your cable modem. So you can not be

confused with your neighbor. In fact you can several different ip addresses,

but only one ip address per MAC address which is hardware based. If you

check, the ip address the cable company assigns to you is different from
what
you computer will report as it's ip address.

It depends. The cable modem is generally at best a bridge, not a router.
It doesn't do address translation. At least with Time Warner and Cablevision.. haven't used any others. If you buy straightforward cable modem service, your computer will do a DHCP request and get an IP lease direct from the provider.. and the cable modem will not have any IP address at all. If you either provide your own router in-between your computer(s) and your cable modem, then it will do the DHCP request to the provider and get assigned the IP address.. and then it will assign non-routable addresses to your computers.
The same is true if you pay extra to the cable company for a combination router/modem. Time Warner wants $50/installation + $5/month for WAP/router.. which is way more than you would pay buying one yourself.. so it makes no sense to get it from them. What you do is get a simple cable modem.. then after the installer leaves, you clone your computer MAC address into the external interface of the router.. because they will lock your IP address to your MAC address.. so that you cannot simply change your IP to some random one and mess with other subscribers on the network.

[#] Tue Feb 17 2009 17:34:49 EST from LoanShark @ Uncensored

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address to your MAC address.. so that you cannot simply change your IP

to some random one and mess with other subscribers on the network.

They do that? That's really evil. Cablevision doesn't do that (although to get a device with a different MAC address to work, you have to power cycle the cable modem.)

What if you buy a new computer? You have to pay to have an installer come back out? That's ridiculous. you should be able to just pick up the phone (which is still evil)

[#] Tue Feb 17 2009 17:51:49 EST from rod @ Uncensored

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It's not really a modem anyway (since it doesn't modulate/demodulate) it's actually just a network adapter. (semantics, I know.)
Anyway, the cable "modem" has it's own MAC - the ISP often uses this to supply service - during provisioning the ISP will add the MAC address to the allowed list of adapters. However, IP addresses are passed out to the device that you attach to the ISP's adapter. If I want a new IP address, I just change my router's MAC address and - viola! new IP. (You may have to power cycle or soft reboot the router.)
The majority of cable modems have an IP address of 192.168.100.1; punching this number into a web browser often shows diagnostic pages. Don't get any ideas though, if your ISP finds you hacking your modem, they'll kick you off or maybe even try to have you arrested.
Conversly, most home routers are 192.168.1.1 or 192.168.0.1; good to know if you ever need to change your config... or someone elses...

[#] Tue Feb 17 2009 17:55:33 EST from LoanShark @ Uncensored

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If you ran your network adapter in promisc mode while connected to, I think, the old LanCity cable modems (pre-DOCSIS), you could see the modem sending out a BOOTP request, I think. You could watch this traffic to figure out your modem's IP, but I never bothered to see if it was hosting any diag pages...

[#] Tue Feb 17 2009 19:43:45 EST from IGnatius T Foobar @ Uncensored

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I'm sure the MAC address binding expires out of their system after a period of inactivity. I know that with Acecape DSL, there's a MAC address lock, but you can unlock it using their web portal or a phone call to the NOC if you're changing around your equipment.

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