Confused About Mac Addresses For Router - MajorGeeks Forum

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Discussion in 'Hardware' started by compudan, Mar 4, 2009.

1. 

   compudan Private E-2

   I apologize in advance for the length of this message. I am probably needlessly complicating a simple question. I have an external Comcast cable modem hooked up to a separate Netgear wireless router. It has two separate MAC addresses for the internal private LAN side and the public WAN side. (The hexadecimal values are the same, except for the rightmost one.) I understand the need for this, and it would be much less confusing if the device was a combination router/modem. In this case, the ISP assigns a different IP address if I plug my computer directly into the modem than if I plug it into the router. (I am not talking about an internal address like 192.168.1.3. I mean a different WAN address like 68.72.124.47. vs. 68.72.123.21) Often the last two octets change, depending on if I use the modem or router directly. Why is this the case? Also, the modem itself has its own MAC address stamped on the underside. I'm assuming the cable company authenticates the account with this MAC address? If the modem has a MAC address, wouldn't it also need its own IP address that is dynamically assigned and separate from the one that was dynamically assigned to the router's WAN side? I guess what I am confused most about is that I thought packets "hop" from place to place. In this case, the packet would start at my computer, hop to the internal side of the router, hop to the external side of the router, and then hop to the modem, then out the cable to the next router. However, when I go to a site like www.whatismyipaddress.com, it gives me the dynamically assigned IP address for the WAN side of the router. Wouldn't the first thing the cable company and outside world sees be the IP (and MAC) address for the modem itself, or are these values masked, and does the cable company see the MAC and IP address from the router's WAN side instead. Thank you

   compudan, Mar 4, 2009 #1
2. 

   Squeaner Specialist

   Your in luck as I work for a Cable Company and run this type of setup. Here's how it works: 1. When you are given a Cable Modem it has to be provisioned with a certain package/speed based upon the HFC Mac Address on that Cable Modem. 2. What that allows is for the Cable Modem to come online and revieve an IP address used for local purposes only. It is not used for any Internet based traffic. 3. When you hook up your Netgear Router behind the Cable Modem, then the Cable Modem acts as a Bridge between your ISP and your Netgear Router allowing all traffic to pass through. 4. Your Netgear Router has a function called NAT (Network Address Translation). This allows it to distrubute one single IP Address from your ISP for all devices that connect to the router. It accomplishes this function by providing IP Address in a different range and automatically takes care of all the transactions for you automatically. Now that we've got down that part, let me answer your question if I haven't already. The 66.xxx.xxx.xxx IP address you see is known as a Public IP Address and is the IP known to the world. With this IP address anyone in the world can talk to your Netgear router while your Cable Modem acts as the bridge. The IP's your computers recieve, 192.168.1.xxx are known as Private IP addresses, and are not routeable in the outside world. It is strictly meant for Internal use only. Hope this helps. Let me know if you have any questions.

   Squeaner, Mar 5, 2009 #2
3. 

   PEBKAC Private First Class

   compudan said: ↑

   In this case, the ISP assigns a different IP address if I plug my computer directly into the modem than if I plug it into the router. (I am not talking about an internal address like 192.168.1.3. I mean a different WAN address like 68.72.124.47. vs. 68.72.123.21) Often the last two octets change, depending on if I use the modem or router directly. Why is this the case?Click to expand...

   Another point to consider: Having a different IP address assigned to the computer vs. the router, when plugged into the cable modem, could also be attributed to the fact that you are probably obtaining your externally facing IP address on a lease from the ISP via DHCP. Most likely, if the lease for the IP address assigned to the router is still good when you swap it out with your PC, the DHCP server will give the computer a different IP address. Once the IP lease for the router expires, it would go back into the pool and be assigned to another device (assuming the router doesn't reappear on the ISP's network within its lease time). That's why if you release the IP address on the WAN connection of the router and renew it, you may not obtain the same IP address from your ISP. Speaking strictly from a corporate network perspective (not a cable modem perspective), another thing which can cause confusion related to a completely different subnet appearing for different devices when plugged into the same network jack is a NAC (Network Access Control) device. Such a device may be configured to identify computers by MAC address and only allow them on that network if the MAC is recognized. If the MAC is not recognized, the NAC can prevent the access of the unauthorized MAC address or throw the computer onto an isolated subnet (VLAN). This helps network administrators prevent end users from doing things like plugging their home equipment into the corporate network or making it more difficult for a would-be hacker to obtain physical access to the local network. (Notice I said "more difficult" and not impossible.)

   PEBKAC, Mar 6, 2009 #3

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