I have inherited an environment where many things don't make sense and I am now in charge of making changes to it and for this I have to understand what the hell is going on before I modify.
Long story short - there is an email server on the LAN and a firewall on the WAN. The WAN IP of the firewall ends in .89. The A record for the email server ends in .90. (The IP pool we have is .89-.93 although only 1 address, .89, is actually plugged in - there is only one line that terminates in the server closet from the ISP).
I am trying to figure out how this is possible. By logic I can conjure, unless the ISP is doing something to redirect traffic from the .90 address (which is not physically plugged in) to the .89 address, nothing addressed to .90 should arrive at .89 - but it does.
I was told by the ISP that they were not doing anything of this kind, but I have run out of other ideas... I only need to know because the IP pool is getting changed (the ISP is switching to a different address pool) and I need to know about all the configuration points to make sure that once the IP change happens, there will be no stale configurations in place and email will flow to the new address correctly after the DNS modification propagates.
Please let me know if there are any other scenarios you can think of that could be happening here. I don't think the firewall could be doing the redirecting when its WAN IP is .89 and therefore shouldn't see any traffic directed to .90, should it?
Thanks in advance,