Understanding how our ISP's directs/routes a 2nd block of IP addresses at our firewall

I recently helped a consultant change out our firewall. We recently moved to a Dell Sonicwall NSA2600.  One of the steps in putting the new firewall in place was configuring the WAN interface the same way it was on the old firewall WAN interface, no surprise there.  With the exceptions of altering some access rules on the new appliance we aren't really have any issues so all is good there.  What I did come to realize is that we have two separate blocks of public IP address or better I should say two blocks of non-contiguous Public IP's.  I'm somewhat new to my company and never game detail attention to all the public IP's so I figured I would use this IP Range finder  and discovered there where two blocks - http://www.csgnetwork.com/ipinfocalc.html.

My question is in that we have the first block that we had assigned to us configured on the WAN interface however nowhere in the firewalls did we have configuration for the 2nd block however it works. Those IP's are used in our Firewall rules and traffic gets into the services we allowed via access rules.    I called the ISP and tried to get some more information on how it works but the tech could tell me that when he logs in our (ISP provided) router/modem he can see the blocks of IP's.   So it's obvious that the ISP is doing something on their end to route traffic to those IP's on their end.  I'm just amazed that we didn't have to specify that 2nd block of public IP's on the existing or another WAN interface on the SonicWall.  Does anyone know how this is working? also what the ISP is doing to direct traffic to those IP's?    I'm sure many people have the same scenario, I'm just looking for a fair understanding because right now I have not clue how to remotely explain how this works.

Thanks EE community!
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This is done in the ISP gateway, routing both ranges of addresses to the one you have configured.

Typically the ISP would do this for you.

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