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Basic Qs: router status page, and mac cloning

Posted on 2006-04-11
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Last Modified: 2010-03-17
First --

I think I understand what IPCONFIG tells me -- what IP has been assigned to my PC's network device, and what is the IP of the device it connects to (the gateway)

So if I go to my router status page (in internetn explorer), and see the following:
     Login Type: DHCP    
     Internet IP Address: 24.5.114.46    
     Subnet Mask: 255.255.248.0    
     Default Gateway: 24.5.112.1

Which IP number above ("internet ip address" or "default gateway") corresponds to the cable modem? And what does the other correspond to?


Second --

I read the following at ee about Mac Cloning:  "it is used when your cable company registers the MAC addreess of the first machine that connects to it, and then won't connect with the router.  you then clone the mac address fo the first machine that connected, and then the cable company thinks that's the machine that is conencted"

WAit -- how can the cable company see into the router at all? How does it see past the modem? I th oguht they just communicate up to the modem, that's it...   And does this issue depend on modem/router  brand/model, or is it a universal issue? (if so, why did I only encounter it recently?)
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Question by:dgrrr
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pjtemplin earned 400 total points
ID: 16435758
First question: your cable modem does not have an IP address.  Your router does, and the cable company's gear has one (the 24.5.112.1 address).  Your neighbors (with cable modems) have PC addresses; all of these are likely in the range 24.5.[112-119].x.

Second question: Ethernet operates at layers 1 & 2.  IP operates at layer 3.  TCP, UDP, ICMP, and others operate at layer 4.  Applications and stuff operate in layers 5-7.  Devices in a common subnet (i.e. your PC and the CC's gear) communicate with each other; your PC does not know how to speak directly to anyone outside its subnet, and hands all off-subnet traffic to its default gateway.  For your PC to exchange traffic with the CC's gear, it must ARP to find out the MAC address it has.  Likewise, the CC's gear must ARP to find out the MAC address your PC has.  To discourage routers, CCs often put enormously high (almost static) timers on their ARP entries.  You countered that by doing a MAC clone.

This is normal, everyday network operation.  Your PC does the same thing to find your router now.
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by:jabiii
jabiii earned 400 total points
ID: 16439018
Your PC is 114.46
Your Gateway is 112.1.
You may or may not hook directly up to the GW you could be going through switches etc.
For Arp.s your normaly (see almost always) should only see ip/mac table for your local network segment (255.255.248.0)
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by:jabiii
ID: 16439022

do an arp -a at a command propt to see the mac's you know.
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Author Comment

by:dgrrr
ID: 16440382
I think I understand better now -- I was confusing several things:
(a) IPCONFIG > IP Address -- this is my PC, from INSIDE MY NETWORK (my side of router)
(b) IPCONFIG > gateway -- this is my ROUTER, from INSIDE THE NETWORK (my side of router)
(c) ROUTER STATUS > Internet IP Address -- this is my ROUTER, from the INTERNET (other side of router)
(d) ROUTER STATUS > default gateway -- this is the cable company. BUT - their far away machines, NOT the modem?



to pjtemplin - question B.1:
What I don't get is -- WHen I REMOVE THE ROUTER, and connect my PC to the modem direclty, the "IPCONFIG>gateway" shown

is yet another value -- 67.164.9.1.  This does NOT MATCH (d) above, which is 24.5.112.1. Why is this different? (aren't both the

"cable company"?)

to pjtemplin - question B.2:
Just to be clear -- although both "router status > gateway" (using a router), and "ipconfig > gateway" (using no router)  both

indicate a gateway that is NOT a router -- in both cases, the IP is not the actual MODEM, but some machine on the cable company

premises -- what you called the "cable company's gear"?

to pjtemplin - question B.3:
I originally thought the cable company checks the mac address of the MODEM. But it sounds like it "ignores" the modem, and

checks the MAC of the first device it connects to AFTER THE MODEM.  Is that right?
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by:dgrrr
ID: 16440409
Jesus. I fucking HATE the way notepad's wordwrap chops up text.  Sorry about the above post format.
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by:pjtemplin
ID: 16440499
Not sure on B.1.

On B.2, your router's gateway (if the router is in) and your PC's gateway (if the router is out) IS in fact a router, and the router that your router or your PC forwards packets to on their way to the Internet.  And no, the IP address is not the MODEM.  

For B.3, they don't care about the MAC of the MODEM.  They only need to figure out how to reach the MAC of your PC (or your router, while it's spoofing as your PC).
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by:jabiii
ID: 16440788
P1, the switch your jacking into might have autmaticly assigned VLANS, and it's modem mac is in a known VLAN, and assigns it as such. Being that all of them are on DHCP, depending on what VLAN you get is what GW you get... although it should change "that" much on one ISP.... just a thought
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by:dgrrr
ID: 16441339
to both:

Hmmm --  I just spoke to comcast (cable), and they said they ONLY access and reference the MAC address of the MODEM -- nothing beyond the modem (not the router or PC).  THis makes sense -- because I can connect both with and without the router using factory settings (no cloning).  Or have I confused things?

And I now understand that both my PC (when used alone) or my router (if used) are finding a gateway on the internet that is actually another router.  cool.

But comcast (1st tier tech support) didn't know why "router status" is showing my different IPs than "IPCONFIG" (without a router)

I hope you guys don't mind if I keep this thread open a little longer, altho you've cleared up a lot.
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by:pjtemplin
ID: 16441608
Whatever.  Your MAC (router or PC) is an essential part in being able to send and receive packets.

DHCP and/or PPPoE takes care of address assignment rather well here.  Might be easier to not worry about what your IP address is.  It is what it is.

Sometimes, it's easier to just say "it works".  
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by:jabiii
ID: 16445082
agree with Pj
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by:dgrrr
ID: 16450900
Hmmm -- but this isn't a "how do I fix it?" post, it is a "how does it work?" post.

Since several unsolved situations seem to have come down to the "what does the router status page say about your modem" question -- what that page actually tells me, and how it relates to the numbers I see under IPCONFIG withOUT the router, is significant to me.  
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by:pjtemplin
ID: 16450923
Understood.  But there comes a point where "how does it work" isn't that important, or at least parts of it aren't important.  There's a lot of magic in PPPoE, DSLoATM, cable modem networks, etc.; some of that magic is simply out of your control/concern.  Throw in some misinformation from the cable co ("they ONLY access and reference the MAC address of the MODEM -- nothing beyond the modem"), and some of us just say "focus on what you can fix and what you can troubleshoot".  In the face of trouble, do this:

ping 127.0.01
ping <NIC IP address>
ping <default gateway>
ping <known remote IP address> (pick a favorite now, while your connection is up)
ping <DNS server address>
ping <known website that responds to pings> (pick a favorite now)
tracert <known website that allows traceroutes> (pick and document a favorite now)

If you can't ping your gateway or beyond from the device connected to the cable modem, call the cable co and work with them.
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by:dgrrr
ID: 16451086
On the phone tonight, Comcast tier 2 was unsure, but guesses that the two different sets of Comcast gateways (24 vs 67) are being assigned because of the differing mac addresses of the PC NIC and the ROUTER...  BUT, at the same time, he confirmed that they ONLY confirm / register the MAC address of the MODEM to provide access... (???)  If that were true, it means they do "peek at" the MAC of the first device on the USER side of the MODEM, and it affects which gateway is used -- but they use the MODEM MAC for ISP / REGISTRATION purposes...  Wonder if that's actually true.
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by:dgrrr
ID: 16451090
DISREGARD ABOVE POST -- sorry, it's for another thread
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Author Comment

by:dgrrr
ID: 16451106
DOH - it is the right thread. You just responded so fast it confused me!  (cool)

Thank you for that list -- I will use it!!!  
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by:jimcowan
ID: 16514892
ok, wow what a mess.

The cable company does not care what MAC address or device is on your side of the modem, however the modem does care, so you must turn a cable modem off for a few seconds, and leave it offlong enough to purge it's memory of the old MAC, (or else it thinks you are trying to get another ip without paying for another connect), when you change the device and therefore MAC connected to it.

Ipconfig gives the ip address of only adapters in your computer, and the router status page gives only the address of the router, which in your case, the external is the internet address, because the cable modem is more or less a bridge rather than a router, but you can also get in to look at moist cable modems with the address http://192.168.100.1 with this you can see the cable modem status page. and notice that 192.168.1.100 is not a gateway even though it is an address of the cable modem internal interface.

The gateway issue with the cable company likely comes from their extensive use of bridging, rather than routing in their networks, and their use of supernetting. Needless to say, just know that the gateway machine at the cable company is assigned, and cannot be changed except by them.

Hope this helps, yell if I can answer anything else
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by:mav7469
ID: 16528004
  I am not sure if this is going to add to the confussion or not, but I will try.  In the "way back" days  ;)  comcast cared how many computers you had connected to thier network.  This was because, unlike DSL which is a direct connection from the ISP to your Home, Cable modems (Comcast) was basically a WAN.  They ran one large cable out to your area and then connected you to the wide area network in your area.  Comcast wanted to get an idea of how much bandwidth you would be using at any particular time, so they limited the connection to one computer by the MAC address.
   Linksys and other Router manufactures foudn a way around this by using MAC Cloning.  This basically fooled Comcast into thinking that you only had one computer (the same computer) connected to the router.  But, behind the router, you could have 200 or so computers.  
  Once Comcast realized that this was happening, The first tried to fight against the router companies.  If you ever read the license agreement for Comcast, it actually still states that you agree to only have one computer attached to eacmcable modem to the house at any one time.  If you had a laptop and a Desktop, you had to give BOTH of those MAC addresses to Comcast and then you had to unplug and replug each device to use it.  There are actually cases of people having thier cable modem service shut off for this back in 1999 and 2000.  
  now, with technology and bandwidth the way it is, Comcast and others have decided that it is not worth the fight.  However, because the original system required MAC addresse authentication, you still can only connect to comcast using certain MAC addresses.  Now however, you can just call Comcast and get a MAC address added to your account.
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