Let's suppose I'm a company and I have purchased a block of 256 (x.x.x.x/24) public IP Addresses from APNIC.

Now a company will have internet purchased from some ISP and that ISP assigns a public IP Address to the main server of the company then they can further NAT it. But now, what is the use of 256 addresses that they have purchased and they can't put those IP Addresses on any of their servers right? if they do they have to be on the same gateway but they are connected to an ISP using their assigned address.

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    They can assign their 254 usable addresses any way they wish. There is no need to NAT unless they have more than 254 devices they wish to connect. – user4556274 Apr 19 '20 at 14:00
  • But they have purchased internet from an ISP so will they use their assigned public IP? how will their packets route? – Javed Ahmadzai Apr 19 '20 at 14:02
  • They will have to ask their ISP to route the address range to their servers. Which not all ISPs do, so maybe they'll have to change their ISP. Or get a different contract. – dirkt Apr 19 '20 at 14:06
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    There is no restriction that an ISP provide a connection routing only one public IP to a location. That is typical for residential accounts, but is not necessary. – user4556274 Apr 19 '20 at 14:06
  • I have done this at clients (not with 256 IP, but a smaller group). All traffic is independent, gateway addresses are different, there is no connection. The IP addresses could easily come from two ISP's and work the same way. – John Apr 19 '20 at 15:33

How a company will have internet purchased from some ISP and that ISP assigns a public IP Address to the main server of the company then they can further NAT it.

That's not quite how it works. Companies typically have one or more routers that connect to the ISP (through some telecom medium). The company and the ISP will agree that the company will advertise their block to the ISP (via BGP) and the ISP will in turn advertise it to the Internet. Sometimes the ISP will advertise the block on behalf of the company and statically route traffic to the company's router, but this is not common.

Since the ISP usually controls the channel between them, the ISP assigns the IP address of the external interface of the router. But as others have pointed out, the company can do as it wishes with their block.

  • Hmm.., now if ISP uses some free block of public IP Addresses for themselves without purchasing from APNIC would that work? ... Google has a Block of 8.x.x.x/8 assigned. Now if they decided to use say 15.x.x.x which is a free block will they encounter any problem? actually I'm trying to understand how things work. and thanks for your answer, I would appriciate if I get some insights on this too. – Javed Ahmadzai Apr 19 '20 at 20:58
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    First, is not a free block. But even if it were, I imagine an ISP that used blocks of addresses not assigned to them would have trouble peering with other providers and therefore trouble staying in business. – Ron Trunk Apr 19 '20 at 21:03
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    I've heard of some companies doing this – but typically their primary business is "sending spam". – user1686 Apr 19 '20 at 21:11

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