# Questions about IP addresses **URL:** \ **Category:** Factual Questions\ **Created:** [May 8, 2008, 10:17pm UTC](https://boards.straightdope.com/t/questions-about-ip-addresses/448618 "2008-05-08T22:17:27Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Eric\_Halfabee](https://avatars.discourse-cdn.com/v4/letter/e/eb8c5e/32.png) [@Eric\_Halfabee](https://boards.straightdope.com/u/Eric_Halfabee)\ **Post date:** [May 8, 2008, 10:17pm UTC](https://boards.straightdope.com/t/questions-about-ip-addresses/448618/1 "2008-05-08T22:17:27Z")
Do users on a cable internet system (like Comcast, for example) share an IP address? ---
**Author:** ![Surbey](https://avatars.discourse-cdn.com/v4/letter/s/4491bb/32.png) [@Surbey](https://boards.straightdope.com/u/Surbey)\ **Post date:** [May 8, 2008, 10:40pm UTC](https://boards.straightdope.com/t/questions-about-ip-addresses/448618/2 "2008-05-08T22:40:19Z")
No. Comcast filters out IP assignment by MAC address. You can sometimes plug a switch and multiple computers into a cable modem and get internet on both but they will eventually turn you off. The only way for this to work is with a router. Sometimes Comcast modems come with routing built in. Other times a user needs to purchase a router, mask the MAC address of the computer assigned to the account, and have everyone (including themselves) plug into the router. Now, you can get multiple IP addresses assigned to a Comcast account. Each computer’s MAC address would be assigned to the account. ---
**Author:** ![Jragon](https://avatars.discourse-cdn.com/v4/letter/j/e19b73/32.png) [@Jragon](https://boards.straightdope.com/u/Jragon)\ **Post date:** [May 8, 2008, 10:41pm UTC](https://boards.straightdope.com/t/questions-about-ip-addresses/448618/3 "2008-05-08T22:41:32Z")
My admittedly shakey Cisco knowlege tells me we’re probably subnetted. I’m guessing people in the same subnet would appear to have the same IP (acutally it would appear with all the nescessary info, i.e. Host IP, Subnet, Your unique ID etc otherwise it probably wouldn’t work), but internally in Cox/Comcast/Whatever’s computer we’re designated with unique IPs in the subnet so they can route the packets to the right computer, rather than bombarding the every computer with uselss data it would now need to process. I’ve actually heard we’ve used up all usable IPs and have to resort to subnetting/subsubnetting/subsubsubnetting etc nowdays. Since I don’t trust myself enough to explain subnetting: > **[Subnet](https://en.wikipedia.org/wiki/Subnetwork)** > > A subnet, or subnetwork, is a logical subdivision of an IP network.: 1, 16  The practice of dividing a network into two or more networks is called subnetting. > Computers that belong to the same subnet are addressed with an identical group of most-significant bits of their IP addresses. This results in the logical division of an IP address into two fields: the network number or routing prefix, and the rest field or host identifier. The rest field is an identifier for a specific host or network int... [QUOTE=Surbey] No. Comcast filters out IP assignment by MAC address. You can sometimes plug a switch and multiple computers into a cable modem and get internet on both but they will eventually turn you off. The only way for this to work is with a router. Sometimes Comcast modems come with routing built in. Other times a user needs to purchase a router, mask the MAC address of the computer assigned to the account, and have everyone (including themselves) plug into the router. Now, you can get multiple IP addresses assigned to a Comcast account. Each computer’s MAC address would be assigned to the account. [/QUOTE] That would make sense, forgot about filtering out by MAC, listen to this one, not me. ---
**Author:** ![Ximenean](https://avatars.discourse-cdn.com/v4/letter/x/aca169/32.png) [@Ximenean](https://boards.straightdope.com/u/Ximenean)\ **Post date:** [May 8, 2008, 11:37pm UTC](https://boards.straightdope.com/t/questions-about-ip-addresses/448618/4 "2008-05-08T23:37:19Z")
Perhaps the OP is asking whether all users of a cable ISP share the same public IP address, i.e. they’re NAT-ed behind it? Some ISPs do do something like that, certainly. Our 3G broadband provider, for example, hands out non-routable 10.x.y.z addresses which obviously must be translated to a routable public address or addresses for us to be able to use the internet. ---
**Author:** ![Eric\_Halfabee](https://avatars.discourse-cdn.com/v4/letter/e/eb8c5e/32.png) [@Eric\_Halfabee](https://boards.straightdope.com/u/Eric_Halfabee)\ **Post date:** [May 8, 2008, 11:59pm UTC](https://boards.straightdope.com/t/questions-about-ip-addresses/448618/5 "2008-05-08T23:59:34Z")
[QUOTE=Usram] Perhaps the OP is asking whether all users of a cable ISP share the same public IP address, i.e. they’re NAT-ed behind it? Some ISPs do do something like that, certainly. Our 3G broadband provider, for example, hands out non-routable 10.x.y.z addresses which obviously must be translated to a routable public address or addresses for us to be able to use the internet. [/QUOTE] This. If I have a cable modem and my next door neighbor has one, too, would we have unique addresses? ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [May 9, 2008, 12:07am UTC](https://boards.straightdope.com/t/questions-about-ip-addresses/448618/6 "2008-05-09T00:07:58Z")
[QUOTE=Eric Halfabee] This. If I have a cable modem and my next door neighbor has one, too, would we have unique addresses? [/QUOTE] Yes – those modems would have different IPs, either directly on the publicly-addressable Internet (pretty common) or on a private cable company subnet with a NAT gateway or some such thing (unusual).