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This is my home network setup:

Internet ---> External IP (cable modem) Internal IP ---> External IP (wireless router) 192.168.20.1 ---> (laptop) 192.168.20.102

Am I correct to assume that the WAN IP / Public IP assigned to me by my ISP is at the external side of the modem or is it on the external side of the wireless router? I am guessing that the External IP of my wireless router has to be on the same subnet of the internal IP of my modem

Where do these 3 IP addresses go into my home network as shown above

  • WAN IP / Public IP (static)
  • WAN gateway (the static Gateway associated with WAN IP)
  • Modem internal IP ( I found it to be 192.168.100.1 a Cisco DPC3008 )

I was thinking more like it goes like this:

Internet ---> WAN IP / Public IP (cable modem) WAN GATEWAY ---> External IP (wireless router) 192. 168. 20. 1 ---> (laptop) 192.168.20.102

Then on this assumptions I'm leaving behind modem internal IP, and External IP of the wireless router would have to be on the same subnet of the WAN gateway which is getting a bit confusing.

What I know for sure is that the External IP of my wireless router should be on a different subnet than 192.168.20.1 so that it can route the packets into another network which is in my case, the internet.

Here is the diagram:

Network diagram

Added by barlop

The item the OP describes as a (cable) modem, is a cable modem router.

Cisco DPC/EPC 3008 has one cable modem connector, and one ethernet port, and also, it is a gateway/router (the document I liked to describes it as such) So it's a cable modem router with one ethernet port. It's sold as a cable modem. (Note that Cisco also do the cisco DPC/EPC2325 which is sold as a gateway but is a cable modem router with multiple ethernet ports)

Additional Info: must read

just wanted throw out a bit of info to make sure everything is crystal, it might or may not sway your answers

  1. Both WAN IP / Public IP (static) and WAN gateway (the static Gateway associated with WAN IP) are present on the WAN tab of the wireless router's admin page which is accessible at 192.168.20.1

  2. On the Routing table of the wireless router's admin page, it shows that destination 0.0.0.0 of Netmask 255.255.255.0 is having the WAN gateway (above) as the determined Gateway using interface eth2.2

  3. Doing a tracert WAN IP, yielded 1 hop/node

  4. Doing a tracert WAN gateway, yielded 5 hops/node (nodes 2 to 4 are all request time out)

side note:

it felt odd that number 3 and 4 suggests that my home network setup is

Internet ---> External IP (cable modem) WAN GATEWAY ---> WAN IP / Public IP(wireless router) 192. 168. 20. 1 ---> (laptop) 192.168.20.102

the reason I thought of this is because tracert means that the packets are being pushed from the wireless NIC of my laptop going outwards into the next node/device

  • still quite lost.. I have known that the cisco DPC3008 could be originally installed without a router in between and directly connects to your computer or laptop by ethernet which means the cable modem itself assigns an IP address to your computer's MAC to communicate with it , whereas in my setup has been with a router in between ever since. Can somebody confirm that my router does acquire its external IP from the modem ? if it is so, would it be in the 192.168.100.0/24 subnet? – amdc Aug 27 '15 at 5:13
  • on a (cable modem)---(computer/Laptop) setup, regarding with the cable modem assigning an IP to the computer/laptop , im not sure if it's the WAN / Public IP or in the range of 192.168.100.0/24, but to think of that as far I know the cable modem does not have a dhcp server but it might be able to give 1 private IP..not sure though – amdc Aug 27 '15 at 5:19
  • Modems don't assign IPs. And modems don't have IPs. These devices are boxes with particular functions. You have to figure out what functions these boxes have. From my experience of cable modems , cable modems tend to just be cable modems, not having additional functionality. – barlop Aug 27 '15 at 22:06
  • Modems don't assign IPs. And modems don't have IPs. These devices are boxes with particular functions. You have to figure out what functions these boxes have. Usually cable modems tend to just be cable modems, though cisco does cable modem routers. Though it could be a current/new thing that devices that function as cable modems nowadays tend to or often function as routers too. Regardless. DHCP servers hand out IPs. Routers have IPs, modems don't. One must isolate the function one is speaking about, within the internet device. – barlop Aug 28 '15 at 10:10
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Provided you didn't place your computer in the DMZ from your router, the closest IP to a public IP would be your WAN facing port on your router. You can log into the router and see what is assigned to it. A Trace route command should also let you know what the IP is on the WAN side of the router. From the DOS prompt try this: tracert 216.58.216.110 This IP is what "ping google.com" returned. If the first hop reported is a 192.xxx IP, then the second one should be the IP your are looking for.

Your computer, in most cases, will get assigned an IP from the Routers DHCP service. Typically in the Class C private 192.xxx.xxx.xxx area.

Are you wanting a public IP on your computer? Your ISP assigns an IP/Gateway to your Cable Modem (typically via DHCP). The Cable Modem generates a 192 network for everything in your home and farms out IP's/Gateways to those device.

  • You write "the Class C private 192.xxx.xxx.xxx area." <-- You're aware classes haven't been used in about 20+ years, right? CIDR(classless....) was introduced long ago and IANA no longer restrict themselves to the subnets decreed by the classful system. And routers no longer use the classful method to determine subnet mask. And it'd always have been a bit misleading to talk of the private addresses as being classful, because the subnet masks of the private addresses don't/have never, corresponded to the classful system – barlop Aug 27 '15 at 21:59
  • Technically true but irrelevant. Class terminology is still used in Cisco courses today (because you have to learn class before you can learn classless), and in this context the OP obviously means "a network of 256 IPs", not "the number of bits used for the network portion of the IP". – goblinbox Aug 27 '15 at 22:33
  • @goblinbox I'm all for learning old stuff but know that it's old and not done anymore. Cisco courses are known to be wrong on this issue, and hence students of cisco are often corrected on it, and are surprised to hear that classes aren't used anymore. And the OP didn't use the term class, this answerer introduced the incorrect terminology of Class. – barlop Aug 28 '15 at 9:42
  • Hi Christopher and Barlop I updated the original post please have a look – amdc Aug 28 '15 at 23:18
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Am I correct to assume that the WAN IP / Public IP assigned to me by my ISP is at the external side of the modem

Yes, the IP assigned by your ISP is associated with the WAN (wide area network, or "external") side of your modem.

or is it on the external side of the wireless router?

No. (We know this because you have a public IP and because you provided a subnet for the network between modem and router.)

Note: It is possible your modem could be configured as a bridge, making your wireless router your gateway device, but that sort of config is incredibly rare these days when nearly all devices do DHCP and NAT.

I am guessing that the External IP of my wireless router has to be on the same subnet of the internal IP of my modem

Correct. The WAN IP of your wireless router is on the same subnet as the LAN (local area network, or "internal") IP of your modem.

In a nutshell, 192.168.100.XXX is a two-device network consisting of the LAN side of your modem and the WAN side of your wireless router.

Then on this assumption I'm leaving behind modem internal IP, and External IP of the wireless router would have to be on the same subnet of the WAN gateway which is getting a bit confusing.

Again, yes, there's a subnet between the modem and the router. There has to be, in order for traffic to flow. (You could get rid of it by bridging your wireless router; then all the devices on the subnet would be getting IPs from the modem instead but there's really no reason to and most wireless routers these days don't even do bridge mode.)

If your setup is typical, this is what's happening:

Subnet 1: Your modem is getting a public IP from your ISP. (This IP might be statically or dynamically assigned, but either way it's a public IP address.) Your modem is on a subnet; it sends traffic to the internet through its gateway IP, which belongs to the ISP.

Subnet 2: Your wireless router is getting a private IP from the modem. This causes the little subnet between modem and router.

Subnet 3: Your wireless router is also a DHCP server. Your client devices -- laptops and tablets and phones and such -- are getting private IP addresses from the wireless router. This is a different subnet than the one between the modem and router.

Your traffic is what's called "triple NAT-ed," meaning network address translation is happening three times. This is totally typical in modern home networks.

You can functionally ignore the little subnet between the modem and router, as no one ever uses those IPs but the two devices themselves, but it is important to know that it's there for troubleshooting purposes.

  • You write "subnet between modem and router." <-- sounds like you've got your layers mixed up, but that you know better and could be more correct in your terminology – barlop Aug 27 '15 at 21:57
  • Subnetting has to do with routing packets from device A to device B on a network; the layer model talks about the physical functions of moving those packets up and down the stack on an individual device. Different things. :) – goblinbox Aug 27 '15 at 22:22
  • You write "Subnetting has to do with routing packets " <-- And guess what layer that/IP/Routing is. It's Layer 3. And guess what layer modems don't deal with - layer 3. Guess what modems don't deal with - routing. So when you're saying "subnet between modem and router." and "Subnetting has to do with routing packets" you're obviously not realizing that modems don't operate at layer 3. – barlop Aug 28 '15 at 0:59
  • Um. The modem under discussion is also a router. The little network between the modem and the wireless router can be called a subnet without offending most people or even being dreadfully inaccurate. Not sure what your point is. – goblinbox Aug 28 '15 at 2:18
  • Hi Goblin box I added some info on the original post please have a look – amdc Aug 28 '15 at 23:13
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I am not quite sure if I understand your question in the right way. I will give it a try:

  • WAN IP/public IP should be associated with the external IP of your cable modem.
  • The WAN gateway then is the internal IP of your cable modem.
  • And the modems "internal" IP is what from your wireless routers point of view would be called the gateway.

So your network setup seems to be the following:

Internet --> (Modem) --> 192.168.100.0/24 subnet --> (Wifi Router) --> 192.168.20.0/24 subnet

Whereas your laptop is in the 192.168.20.0/24 subnet and the Modem and the Wifi-Router are the only two devices within the 192.168.100.0/24 subnet.

I hope this was, what you wanted to know? Best regards!

UPDATE:

More regarding to your posts title: So the external IP of your wireless router is NOT your public IP address. It is an address of your internal 192.168.100.0/24 subnet through which it connects to your modem.

UPDATE2:

Here are some updates referring to your comments on this answer:

1st) Your Modem should not have a DHCP server. Instead it should be a DHCP client that requests network configurations from your ISPs DHCP server (I presume your Modem gets its external IP this way because you stated the keyword "DHCP" within the output of your modems DOCSIS status and DOCSIS is the cable protocol through which your Modem builds its upstream and downstream channels in order to connect you to the internet).

2nd) I could imagine that the internal interface of your modem and the external interface of your wifi router share the same IP address, which is to say they are bridged.

UPDATE3:

Hi amdc. Hereby I would like to answer your last question you asked in the comments: Referring to my update2 you have a great chance, that the external interface of your modem requests its IP assignment from an external DHCP provided by your ISP (for me it is the same issue). Check all the IP adresses of your setup in order to find out, whether the bridging theory in turn is right: But your last update you made suggests, that your cable modem acts as network bridge with your public IP as its one and only address. Then the WAN-interface of your wifi-router would be in the same logical network as your public IP (that means: it differs only in the last octet) and your home network (192.168.20.xxx) would be situated behind a NAT...

  • Hi, when I do route print command on my laptop it is the Internal IP of the wireless router which is shown as my gateway - 192.168.20.1 , by the way just to let you know, the WAN gateway which you believe to be on the Internal side of the cable modem (which id like to think it is so) is not in the range of 192.168.100.0/24 network, it is slightly different than my WAN IP, differing only by the last octet (x.x.x.1) . – amdc Aug 26 '15 at 14:00
  • I am sorry, in my third point I meant "wireless router" and wrote "laptop". But I am confused as you are. Can you print the full output of your route tracing command (tracing the route for example to your public IP or google.com or so)? – fragwürdig Aug 26 '15 at 14:12
  • oh i see im getting to your point, – amdc Aug 26 '15 at 14:35
  • i think its making sense that the WAN gateway is my cable modem's internal IP and also is my wireless router's gateway as it can be shown as the gateway in my router's routing table when using the admin tool at 192.168.20.1. I feel I'm getting quite close to the real thing.. thanks, but who assigned the 192.168.100.0/24 subnet for my wireless router? My Cisco DCP3008 cable modem doesn't have a dhcp server – amdc Aug 26 '15 at 14:47
  • Ok wait does it mean that my cable modem does have a dhcp server? cause i entered the admin tool for DCP3008 using 192.168.100.1 and there are no relevant links/buttons to a DHCP server , just status and log off, well on the status it says DOCSIS Downstream Scanning: Completed DOCSIS Ranging: Completed DOCSIS DHCP: Completed DOCSIS TFTP: Completed DOCSIS Data Reg Complete: Completed DOCSIS Privacy: Enabled – amdc Aug 26 '15 at 14:54
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I really appreciate everyone's input though after further research I finally found a way to understand how my home network works, if someone could post the image I have created from visio in here http://postimg.org/image/qyp8ag54d/

In this setup the cable modem acts as a Network Bridge device which is responsible for making your Local area network as one with your ISP. The static WAN Gateway address provided to you by your ISP is sitting at the ISP's CMTS/UBR network interface (catV segment) which is given a public IP address in the 10.x.x.x.x or 172.x.x.x.x range as a means of sending packets on your cable modem that as a DHCP client has requested a private IP address in that same range

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