Is it possible to connect my private network to the ISP router without a Network Address Translator (NAT) feature enabled on my router? As I understand, in a single-homed scenario connection to ISP, ISP router defines a static route on its router and I need to define a default gateway on our router and advertise this gateway to all members of our network.

I think without the NAT feature on my router, this scenario doesn't work, no?

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    "I need to define a default gateway on our router and advertise this gateway to all members of our network." No, your router is the gateway for your network. Your router will probably have a statically defined default route, and possibly other routes either statically defined or from a routing protocol with your ISP. – Ron Maupin Jan 30 at 13:36
  • What exactly are you trying to do?? – Leo Jan 31 at 6:56
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    You do not configure a default gateway for the router. You can configure and/or receive through a routing protocol one or more default routes for the router. See this answer for an explanation. – Ron Maupin Feb 2 at 8:07
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    @A.A, "I wanted to know without BGP and without NAT, could I connect to my ISP or not" Yes, of course you can if your ISP gives you some public addresses. You can route traffic with public addresses on the public Internet, and you do not want to use NAT on public addresses. You could simply address your hosts with the public addresses and set a default route in your WAN router. Businesses do this all the time. Your company simply needs to buy some public addresses from its ISP. – Ron Maupin Feb 2 at 8:14
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    "Isn't it necessary to run the routing protocol as the same that the ISP network is running?" That is true only if you have provider-independent addressing that you must advertise to the ISP. If the ISP supplies the public addressing, it already knows about it and is advertising it on the public Internet. The AS has nothing to do with that. You need an AS to peer with another company using BGP., but if you do not need BGP, then you do not need and AS, or you maybe could use a private AS if it is just the one ISP. – Ron Maupin Feb 2 at 16:10

Without NAT, you can't use private IPv4 addresses in your network (with Internet connectivity). You'll need to get a sufficient IP address range from your ISP, at least one address for each device with Internet access.

You'll still need a router but it could route public-to-public without any need of NAT. Note that this is the standard way to connect IPv6.

Any private-to-public or public-to-private routing requires NAT for IPv4. (Alternatively, a proxy could be used for many protocols but from the addressing perspective, there isn't much difference.)

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    ISPs typically offer this service under the name "business IP" or "static IP addresses". It often comes in multiple flavors (e.g. a /30 IPv4 prefix = 1 static IP address, or a /29 IPv4 prefix = 5 static IP addresses, or a /28 IPv4 prefix = 13 static IP addresses, etc.) It is significantly more expensive than "consumer IP" or "dynamic IP addresses". – Bruno Rijsman Jan 30 at 13:54

There is absolutely no need to use NAT or private addresses to connect a network to the Internet. NAT was only designed as a short term solution to work around the lack of IPv4 addresses (and yes: NAT is old. People noticed that we are running out of vintage IP (IPv4) ages ago).

There are two different scenarios, it doesn't matter if they are vintage IP or IP (IPv6)

a) Your ISP provides you with a large enough range of public IP addresses. BGP might be used here but it's not necessary.

b) You have your own public IPs. Here the use of BGP is more common but also not absolutely necessary.

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There are two main differences between a "with NAT" and a "Without NAT" scenario.

  1. With NAT you can use anything from the huge range of private IP addresses, without it you are limited to the public addresses you are allocated.
  2. Your ISP must allocate a block of IPs and route it to your router.

How hard this will be depends on how big a block you want and what type of internet connection you are buying. If you are buying a broadband package from a cheap ISP then don't expect to get an IP block. Some more expensive broadband providers will provide blocks, and if you are buying a leased-line or similar service then a block would be expected.

(the above all applies for IPv4, for IPv6 even the smallest customers tend to get blocks, unfortunately they are sometimes dynamic blocks which makes them rather more annoying to use)

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