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Say a router is connected to two devices. Now the router will give those two devices different private IP addresses. The router itself has a private IP address (which serves as the default gateway for those two devices). The router also has a public IP address.

I understand the difference between public and private IP addresses.
But every now and then I see the term local IP address. This is sometimes used in context with private IP addresses.

So I'm confused if they are different or the same terms (private and local IP address). Are they synonyms? If they're different then how so?

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  • Every router interface is in a different network because routers route between networks. If two hosts directly connect to a router on different interfaces, then they are in different networks. Also, there is no real difference between public and private addresses. It is an artificial difference that the ISPs will not route private addresses on the public Internet, but IP has no concept of public or private addresses. To IP, they are simply addresses with no distinction.
    – Ron Maupin
    Jun 28 at 12:31
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    This answer, this answer, and this answer, discuss the Local addresses in a Cisco router.
    – Ron Maupin
    Jun 28 at 12:50
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So, let's say you have 2 interfaces on the router, with the 2 IP addresses. Let's say 192.168.1.1/24 on the first one and 176.16.1.1/24 on the other.

Presuming you're on a Cisco router, if you issue the command "show ip route" you'll see an output which shows the 2 IPs labeled with an L and a C. L stands for Local, and C for connected. Meaning that the 2 IPs are connected and locally significant for that router. Local IP address are the ones you have connected on that specific device. They can be either private or public, makes no difference.

Hope this answers your question.

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  • thank you for answering. What do you mean by Local IP address are the ones you have connected on that specific device. They can be either private or public, makes no difference. What is the specific device? A device connected to the router (like a mobile)? If yes, how can it be public. Please correct me if I'm wrong, but aren't devices(mobile) connected a router have only private IP address of that router?
    – Nicks101
    Jun 28 at 11:00
  • @Nicks101 Local, as in the device you’re actively on (the router itself).
    – Jesse P.
    Jun 28 at 11:37
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    The specifc device is the router itself in this case, as @JesseP. said. Local IPs are the ones locally significant on the router, meaning IPs that are actually configured on the router's interfaces. I.E. interface eth1/0 on router1 has IP 192.168.1.1/24, ergo 192.168.1.1 is a local IP address. Same goes on IP 176.16.1.1/24 on interface eth1/1: 176.16.1.1 is another local IP address. If you have an interface eth1/2 configured with a public IP address, that specific IP is automatically becoming a local IP address. And so on...
    – Giargia
    Jun 28 at 11:52
  • Okay. So overall, all (2) IP addresses in a router are local IP addresses.
    – Nicks101
    Jun 28 at 12:39
  • This answer, this answer, and this answer, discuss the Local addresses in a Cisco router.
    – Ron Maupin
    Jun 28 at 12:45
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A local address doesn't have a strict definition, but in this context it usually means an address on a subnet directly connected to the router. It could be public or private.

While it's very common to use NAT to translate private addresses into public, that is not the only use. In enterprise networks, sometimes we translate private to private, or public to public.

You should understand that devices like home routers (off topic here BTW) have several functions in addition to a router all in one box. In your case, it appears to have a DHCP server and probably a firewall too.

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    I see. It's called local in the context of the direct connection with it.
    – Nicks101
    Jun 28 at 12:41
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The adjectives "private" and "public" refer to IP addresses belonging to different ranges, with different purposes and rules of usage. Any private or public IP address may or may not be assigned to an interface of any given machine.

On the other hand, the adjective "local" is relative to a specific machine. It refers to IP addresses that are directly connected to a specific machine. The description of "local" must always be in the context of a particular machine. That machine's local IP address(es) may or may not be private or public IP address(es). It could also be possible that it has some private and some public IP addresses.

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  • Thank you for answering. Reading this, I have a follow-up question. This might be a little off-topic. Say a device named X (a laptop) has multiple local IP addresses. Since the local IP address can be public, then X is connected directly to a public address. Is this right? From what I've learned so far is that X is connected to the router. Router gives X an IP address. The router sets its own private IP address as the default gateway for X. X does not know about the public IP address of the router.
    – Nicks101
    Jun 28 at 14:47
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    Yes. Because of IP address shortages (with IPv4 specifically), many devices are configured with private IP addresses and use NATs, to conserve IP addresses. But this is more of a policy matter than technology matter. So, it is certainly possible that you could get a dedicated public IP address from your ISP and directly assign it to an interface of device X. You just normally have to pay more for such dedicated IP addresses. So in that case, you would have a local IP address that is also public. Jun 28 at 14:52
  • I see. Now it makes more sense. This dedicated public IP address was the term I was missing. I'll learn about it.
    – Nicks101
    Jun 28 at 15:20
  • Ah, yes, you may see the phrase "dedicated IP address" when ISPs try to sell that to you. Actually, even between public and dedicated public IP addresses, there can be a distinction. Imagine your ISP has a block of public IP addresses to lease out (a DHCP pool) to home routers. These are public IP addresses that are globally routable, but the next time your home router reboots, it may pick up a different public IP address from the pool. Jun 28 at 15:35
  • Can two devices (say a laptop and a mobile) be connected to the same dedicated public IP address? If yes, then how does it know which device to forward the response to?
    – Nicks101
    Jun 28 at 15:38

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