IP address vs MAC address

Ok somebody explain this to me.  I understand how IP addresses are 
issued(well sort of...lol)  Is it one IP address to one MAC address?(not 
accounting for non routable IP space)  I understand there can only be one 
WAN facing IP address per machine/MAC/NIC/etc realizing multiple NICs 
possible on one computer.   If that is the case then how can say a Comcast 
DHCP server keep from issuing an IP to a MAC address that another provider 
has already issued an IP for?   In this case perhaps someone has made up a 
MAC address to use in their router for cloning purposes.   Or does it really 
matter? 


0
Jbob
2/23/2006 4:22:12 AM
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On Wed, 22 Feb 2006 22:22:12 -0600, Jbob wrote:

> If that is the case then how can say a Comcast DHCP server keep from
> issuing an IP to a MAC address that another provider has already issued
> an IP for?   In this case perhaps someone has made up a MAC address to
> use in their router for cloning purposes.   Or does it really matter?

It doesn't really matter.

AIUI, MAC addresses are only relevent within a network segment and are not
propagated over network boundaries. AFAIK there isn't any technical reason
why two NICs can't have identical MAC addresses provided they are not on
the same network. It shouldn't "break" anything.

HTH
HT
0
Horseradish
2/23/2006 5:10:52 AM
On Wed, 22 Feb 2006 22:22:12 -0600, Jbob wrote:

> Ok somebody explain this to me.  I understand how IP addresses are 
> issued(well sort of...lol)  Is it one IP address to one MAC address?(not 
> accounting for non routable IP space)  I understand there can only be one 
> WAN facing IP address per machine/MAC/NIC/etc realizing multiple NICs 
> possible on one computer.   If that is the case then how can say a Comcast 
> DHCP server keep from issuing an IP to a MAC address that another provider 
> has already issued an IP for?

Each device with a MAC address is supposed to have a unique MAC - there
should be only one device with that specific MAC address in the world.  I
do have a couple of 3Com PCMCIA cards with identical MACs to disprove that
theory but I believe that was a manufacturing error and very rare.

In any case, ARP (the protocol that dynamically deals with IP to MAC
mapping) is used only within a subnet, and in a normal configuration
doesn't make it past the first router. One Comcast router doesn't care
about MACs behind another Comcast router, never mind the MACs behind
routers of another provider.


> In this case perhaps someone has made up a MAC address to use in their
> router for cloning purposes.   Or does it really matter?

If there was another identical MAC on the same subnet, both devices
would likely have communication problems.
0
Snowbat
2/23/2006 5:16:22 AM
>"Snowbat" <snowbat@geocities.com> wrote in message 
>news:pan.2006.02.23.05.16.17.839035@geocities.com...
> Each device with a MAC address is supposed to have a unique MAC - there
> should be only one device with that specific MAC address in the world.  I
> do have a couple of 3Com PCMCIA cards with identical MACs to disprove that
> theory but I believe that was a manufacturing error and very rare.
>
> In any case, ARP (the protocol that dynamically deals with IP to MAC
> mapping) is used only within a subnet, and in a normal configuration
> doesn't make it past the first router. One Comcast router doesn't care
> about MACs behind another Comcast router, never mind the MACs behind
> routers of another provider.
>

Ok thanks to both of you.  I didn't know about the subnet thing.  I was 
reading a post from a user in another forum that had sold his router on 
Ebay.   He cloned the MAC of the one he sold into his new router to keep the 
same IP.   When the buyer(Oregon) put the router he purchased online 
apparantly it was given the same IP as the seller(Illinois) was using and 
the sellers new router lost connectivety.  The buyer was a Comcast customer 
as well.   So this kinda made him question whether DHCP was a regional or 
national thing.  In this case it appears that it does make a difference, at 
least with Comcast. 


0
Jbob
2/23/2006 5:55:22 AM
On Wed, 22 Feb 2006 23:55:22 -0600, Jbob wrote:

> Ok thanks to both of you.  I didn't know about the subnet thing.  I was 
> reading a post from a user in another forum that had sold his router on 
> Ebay.   He cloned the MAC of the one he sold into his new router to keep the 
> same IP.   When the buyer(Oregon) put the router he purchased online 
> apparantly it was given the same IP as the seller(Illinois) was using and 
> the sellers new router lost connectivety.  The buyer was a Comcast customer 
> as well.   So this kinda made him question whether DHCP was a regional or 
> national thing.  In this case it appears that it does make a difference, at 
> least with Comcast.

The MAC address is used for layer 2 communications, the IP address is used
for layer 3 communications. Some day, I will actually learn what that
means; at least in more detail than I can currently parrot!

In a dynamic IP address environment, a DHCP router uses ARP traffic at
layer 2 to find out which devices have, or need IP addresses. This requires
using the MAC address. The topology of a given network is not, necessarily,
geographically limited; so a subnet can include different geographic
regions. Hence, it is entirely possible for a device in Illinois, and
another in Oregon, to have the same MAC address; along with the problems
related to having two devices with the identical MAC address on that
subnet.

What that seller could have done was to clone the MAC address of his new
router into the router he sold, as well as cloning the old one into the
new. Odds are, his buyer would never have known, and everything would have
been hunky-dory. Unless he sold it to a guy like me, who would have started
by performing a factory reset to put the router into a pristine state
before configuring it to my specifications. I am not one to clone a MAC
address unless it is absolutely necessary; and I have some old NICs left
over from my MS DOS 6.22/MS Windows 3.1 days from which to get a MAC
address, if I needed one.

In general, if you are the sort for whom bad luck is the only sort of luck
one has, you should only use a MAC address of a device you control.

-- 
Norman
~Oh Lord, why have you come
~To Konnyu, with the Lion and the Drum
0
Norman
2/23/2006 7:20:36 AM
Thanks Norman.....what I do is to clone the MAC address on one of my 
machines to the router.  That way if I have to bypass the router for any 
reason, I'll still get the same IP if I use the same system with the cloned 
NIC.  Not that it really matters.  Like you have have spare NICs just laying 
around.

Sometimes I am afraid of changing a MAC out of fear of getting a different 
IP that has been targeted badly from an old user.  ;-)  I'm not sure if 
there really is any good reason to get a different IP from your ISP though 
except for my previous example.  Maybe after months and months of my son's 
P2P traffic it might be reasonable to go for a new IP just to get your old 
IP off so many the sharing lists. 


0
Jbob
2/23/2006 7:38:21 AM
On Wed, 22 Feb 2006 23:55:22 -0600, Jbob wrote:

> Ok thanks to both of you.  I didn't know about the subnet thing.

May be more accurate to say network segment, as indicated by
Horseradish Tree, depending on the configuration.

> I was reading a post from a user in another forum that had sold his
> router on Ebay.   He cloned the MAC of the one he sold into his new
> router to keep the same IP.   When the buyer(Oregon) put the router he
> purchased online apparantly it was given the same IP as the
> seller(Illinois) was using and the sellers new router lost connectivety.
>  The buyer was a Comcast customer as well.   So this kinda made him
> question whether DHCP was a regional or national thing.  In this case it
> appears that it does make a difference, at least with Comcast.

I'm not familiar with Comcast's setup but maybe he forgot to delete his
account settings from the old router and their system terminated the
oldest login when a new one appeared for that account?
0
Snowbat
2/23/2006 10:21:04 AM
On Thu, 23 Feb 2006 01:38:21 -0600, Jbob wrote:

> Sometimes I am afraid of changing a MAC out of fear of getting a different 
> IP that has been targeted badly from an old user.  ;-)  I'm not sure if 
> there really is any good reason to get a different IP from your ISP though 
> except for my previous example.  Maybe after months and months of my son's 
> P2P traffic it might be reasonable to go for a new IP just to get your old 
> IP off so many the sharing lists.

There is no advantage, to the customer, of dynamic IP address assignment
over static assignment. There appears to be a cost savings to the ISP; but
I am not sure I understand the matter completely.

OTOH, if one does get a truly static IP address, and not just some "sticky"
DHCP assignment, as it typical of cable, it would only be advantageous if
one could get a rational PTR record on that IP address. In my case,
something like, 'wino.aosake.net', instead of the ISP assigned name (which
is, 'adsl-71-132-19-202.dsl.sntc01.pacbell.net', in my case).

-- 
Norman
~Oh Lord, why have you come
~To Konnyu, with the Lion and the Drum
0
Norman
2/23/2006 9:44:11 PM
On Thu, 23 Feb 2006 13:44:11, Norman Miller wrote:

>On Thu, 23 Feb 2006 01:38:21 -0600, Jbob wrote:
>
>> Sometimes I am afraid of changing a MAC out of fear of getting a different
>> IP that has been targeted badly from an old user.  ;-)  I'm not sure if
>> there really is any good reason to get a different IP from your ISP though
>> except for my previous example.  Maybe after months and months of my son's
>> P2P traffic it might be reasonable to go for a new IP just to get your old
>> IP off so many the sharing lists.

>There is no advantage, to the customer, of dynamic IP address assignment
>over static assignment. There appears to be a cost savings to the ISP; but
>I am not sure I understand the matter completely.

There was a cost saving in the 'mainly dial-up' days for some ISPs, but
IMHO an ISP that's any good throws in at least one static IP these days.

>OTOH, if one does get a truly static IP address, and not just some "sticky"
>DHCP assignment, as it typical of cable, it would only be advantageous if
>one could get a rational PTR record on that IP address. In my case,
>something like, 'wino.aosake.net', instead of the ISP assigned name (which
>is, 'adsl-71-132-19-202.dsl.sntc01.pacbell.net', in my case).

I guess it's all down to which ISP you choose - if you have a choice
that is.  Here in the UK nearly all ISPs are available country-wide, so
one can pick and choose according to what one wants in price and
features.  Interestingly some of the cheapest ISPs dole out static IPs
at no extra cost, and will even thrown in a domain to boot.  You have to
pay a bit more to find one that'll let you play with your own zone files
though.  Sometimes the best way is to use one ISP purely for
connectivity, and a specialist ISP[1] for nearly everything else.

[1]     An Internet Service Provider can provide all sorts of services,
        which don't have to include connectivity.  An example is
        http://gradwell.com who I can commend as a very satisfied
        customer.

-- 
Jim Crowther.    "Life is not a journey to the grave with the intention of
arriving safely in a well preserved body, but rather to skid in broadside,
thoroughly used up , totally worn out and loudly proclaiming;
WOW!!! What a ride."                          "It's MY computer!" (tm SMG)
0
Jim
2/23/2006 10:47:46 PM
Norman Miller wrote:
[snip]

> The MAC address is used for layer 2 communications, the IP address is used
> for layer 3 communications. Some day, I will actually learn what that
> means; at least in more detail than I can currently parrot!

Ulysses Black has a book that explains it well.

Prentice Hall
"OSI: A Model for Computer Communications Standards
Uyless D. Black, Information Engineering Institute
ISBN: 0-13-637133-7
Publisher: Prentice Hall
Copyright: 1991
Format: Paper; 640 pp"
http://tinyurl.com/qqrpo
http://vig.prenhall.com/catalog/academic/product/0,1144,0136371337,00.html

Amazon
http://tinyurl.com/qlc7u
http://www.amazon.com/gp/product/0136371337/002-1603120-1007220?v=glance&n=283155

My understanding is that the OSI model is used differently depending upon which
side of the border you are on. I'm not talking about your nationality. 8^)
While a LAN uses the MAC for addressing layer 2 (data-link), the WAN goes into
areas I only am aware of from book knowledge like encapsulation of ATM or
frame-relay. I've seen reference to layer 1 and to layer 2 for these concepts.

Aren't WAN links addressed by the switch outside the demarcation?

[snip]

> What that seller could have done was to clone the MAC address of his new
> router into the router he sold, as well as cloning the old one into the
> new. Odds are, his buyer would never have known, and everything would have
> been hunky-dory. Unless he sold it to a guy like me, who would have started
> by performing a factory reset to put the router into a pristine state
> before configuring it to my specifications. I am not one to clone a MAC
> address unless it is absolutely necessary; and I have some old NICs left
> over from my MS DOS 6.22/MS Windows 3.1 days from which to get a MAC
> address, if I needed one.

Why?

The only reason I can see is to avoid updating static ARP or ACL entries. I
wouldn't think a resale store would give a <beep> about that.



-- 

Dave Keays
0
Dave
2/25/2006 11:34:12 PM
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