How exactly do "Virtual WAN IPs" work, and is that what I'm seeing?

Hey there,

Been trying to find the right terminology and I'm a little stumped, as well as my direct boss doesn't have an answer.

I work for an MSP and some of our clients have been having Firewall or internet upgrades, and while I'm not sure if my experience today was based off of the same thing I'll describe, I know a previous experience or two was.

So our clients usually have Comcast. They have Static IPs. We often see that their WAN IP is something like 73.x.y.z when remote connect software reaches out to us, or when someone is on-site and checks Whatsmyip, and that's what we use to configure our site to site VPNs. These are normally persistent through firewall firmware updates. But sometimes when we perform an upgrade or they get an internet upgrade, we are given IP config information that is very different, usually a 50.x.y.z WAN and Gateway. Example, I recently did a firewall install at a new location that had a static IP through Comcast. Their WAN IP was 73.x.y.z, but over the phone, when discussing configs with the rep, I was told that their IP and Gateway IPs were 50.x.y.z. I was told that the 73.z.y.z was some sort of "Virtual IP" but they couldn't explain any further. After working with the rep, Whatsmyip showed the 50.x.y.z address I had used on the firewall and expected to see, rather than the previous 73.x.y.z.

I don't know a ton about SDWAN, or if that is what is the root of all this, but multiple times we've had to update configurations that we weren't expecting to update, and don't always have someone with the credentials to perform adjustments at our Co-location. I saw another IP go from a 73. to a 50. today after an upgrade, which has broken a VPN I can't fix with my credentials, while the user was told they kept their same static IP, so is there a misunderstand on my part here or something more complicated on the ISP side of things?

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Thank you,