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I have studied networking only a little before and the concept of IP addresses is still confusing to me. If I've understood correctly, IP addresses are specific to the network that I'm currently in, that is, if I have a certain IP address, someone else on another network can have the same IP. When my data needs to be passed to a receiver that is on another network, my network routes my data to an entity that can pass my data to the another network (like my router).

Now, when I use my router to connect to the Internet, my router receives an IP address from my ISP that is then used within the network of the ISP to make sure that my data comes to me instead of other clients of my ISP, correct?

So does my data go to my ISP, through some kind of central hub that has its own IP address(es) as part of some bigger network that connects my local ISP network to, say, the American internet provider that hosts whatever service my data is intended to go to?

But there are sites on the Internet that tell me my IP address. Is this IP address the address that I have been given by my ISP? It seems to be something related to my ISP, as the geolocation services can tell my location and the name of my ISP. But I don't understand how this is possible, since I thought that the network of my ISP is part of some larger network, which could be a subnetwork of another, etc.

If somebody could clarify how IP addresses work I would be very grateful. I fear that my question is quite broad, but I don't know how to ask this another way.

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5 Answers 5

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if I have a certain IP address, someone else on another network can have the same IP"

No, definitely not. An IP address must be unique and it must fit into the subnet you're in.

Your router's external IP address is issued by your ISP (usually temporarily). It is to this IP address that all your requests need to be translated to and the answers back from (source NAT).

This external IP address has been issued - in a large block - to your ISP by your regional Internet authority. It is unique world-wide (unless your ISP also uses NAT) and required to send back data to you.

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    But when I turn on my router and connect to it with my PC, my PC gets an IP address of 192.168.(something). When I go to work and check my IP, it is something similar. So I thought that the same applies to higher levels also: As my router and the router at my work are part of another (higher) network, they could also have IP addresses like that as they are now part of another network. It appears that I don't really understand network at all..
    – S. Rotos
    Commented Sep 4, 2017 at 19:01
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    There are IP address ranges reserved for private use (192.168.0.0/16, 172.16.0.0/12, 10.0.0.0/8) that cannot be used on the Internet. However, they can freely be used inside private networks. For them to use the Internet, their (source) address must be translated by a NAT router - it's replaced by the router's external, public IP on the way out and on returning data, this process is reversed. This allows you to provide Internet access for several (in fact several hundred or even thousand) computers through a single, public IP address.
    – Zac67 ♦
    Commented Sep 4, 2017 at 19:07
  • Okay, now it makes much more sense. I'm accepting your answer.
    – S. Rotos
    Commented Sep 6, 2017 at 11:21
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The quick simple answer to your question is yes, the IP you see on those sites is a generally a globally routable IP address that your ISP uses for your connection to/through them. As for how information can be ascertained about your location, as part of the assignment of that IP address, the ISP can use DHCP options (option 82 and it's sub-options) to log the location data that is associated with your connection. That information is stored and used for these lookups.

And as far as can someone have the same IP on another network, yes. RFC1918 defines private address space to utilize within a private network.

10.0.0.0/8
172.16.0.0/12
192.168.0.0/16

There is nothing stopping you from using this address space and another network using the same. The key here is that in order for these private addresses to communicate with something non-local (e.g. an Internet Service) they will need to be translated to a globally routable IP (the one your ISP uses) using NAT.

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    It's 192.168.0.0/16, mate. ;-)
    – Zac67 ♦
    Commented Sep 4, 2017 at 19:11
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    force of habit writing that one out for home networks :0
    – HostBits
    Commented Sep 4, 2017 at 19:19
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Senario 1:

Let's assume your computer is within a LAN (let's say LAN1), and your computer's IP address is 192.168.0.100. Your friend is in another LAN (Let's say LAN2) with the same IP 192.168.0.100. Now you want to chat with your friend. How will it happen ?

Let's say, Your computer is connected to a router (Router1) with the IP x.x.x.x. Your friend's computer is connected to another router(Router2) with the IP y.y.y.y Router1 is connected to Router2. It means LAN1 (Network 1) is connected to LAN2 (Network 2) via Router1 and Router2. The data will be forwared/received to/from another network with the help of these routers.

Data will be transfered tin this way

192.168.0.100 <--> x.x.x.x <--> y.y.y.y <--> 192.168.0.100

Router1 and Router2 will understand the IP address from it's routing table.

Switch, bridge, routers, gateways, NAT are responsible for the data transfer.

I would suggest you to read functions of 7 layers in OSI model. Also read the devices that are involved in data transmission.

But within a single network, IP address is unique. It means, if the IP address (192.168.0.100) is assigned to you no other person can have the same IP address (192.168.0.100). Because IP address is unique within a single network.

But other networks can have the same IP address.

Senario 2:

Your computer is connected to a proxy server ( Local IP: 10.10.80.12). This proxy server has two IP addresses(Local & External). This proxy server is connected to Internet. It's external IP is 212.25.28.65 which is given by the ISP. Your computer's IP address is 10.10.80.111.

Your friend's computer is connected to another proxy server ( Local IP: 10.10.80.13). This proxy server has two IP addresses(Local & External). This proxy server is connected to Internet. It's external IP is 121.15.27.65 which is given by the ISP. Your friend's computer IP address is 10.10.80.111.

Data transmission will happen in this way-->

(You) 10.10.80.111 <--> 10.10.80.12 (212.25.28.65) <--> Internet <--> 121.15.27.65 <--> 10.10.80.13 <--> 10.10.80.111 (Your Friend)

In this situation if you type 'what is my IP address' in google you will be seeing your IP is (212.25.28.65). Because this is the external IP.

I'll suggest you to read about DHCP and Proxy servers.

Senario 3:

Your computer has one IP address 212.25.28.65 which is given by the ISP. Your friend's computer IP address is 121.15.27.65 which is given by the ISP.

Data transmission will happen in this way-->

(You) 212.25.28.65 <--> Internet <--> 121.15.27.65 (Your Friend)

Hope this will help you.

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All ISP again interconnect regional to globally for entire IP networks to interconnect. These interconnections are only for publicly routable IP Address otherthan RFC1918.

By this global interconnections its possible to reach othernetwork globally. Catch here is its not feasible to use public IP addresses for all hosts both capacity and also security wise. To overcome this NAT concept has been developed and mostly been adapted by regional, global ISP and also in fact enterprise environments. Ideally NAT concept is your private IP Address ranges RFC 1918 are NAT'd to your ISP routable IP, there are many variations based on limitations of each variation a new variation has been introduced. NAT basically configured on gateway device e.g. routers, firewalls, home based modems etc. maintains sessions tables initiated by private IP address to route your request to outside routable world and then send response back to request ( source) IP host.

By this when you check Whatsmyipaddeess kind of sites IP showing is your ISP routable IP address and that is common by default. It works based on HTTP headers reading.

You can read on BGP concepts like Peering, AS number to understand global ISP exchange route information and also NAT concept (NAT wiki page could be good starting point)(https://en.wikipedia.org/wiki/Network_address_translation) solving challenges of routable IP address capacity and also improving security posture.

HTH

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Geo location of IP address has many factors. Geo location of IP address is determined where the IP address block is registered to. If the block is registered to specific ISP and you are getting IP address from that ISP then you are most likely to see geolocation of that ISP details. You can confirm what is my ip and IP location details via websites like http://whatismyip.live etc.

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