I am guessing that this isn't really about sequential IP's. There's probably one of two things happening here:
1: You can't use the same IP, because the application uses something very proprietary that associates a unique user with a unique IP - When you have multiple people on a single IP (as normally occurs on a SOHO router running normal NAT) it gets confused who is who.
2: The app is using a protocol that doesn't support more than one user across a NAT. A common example of this is ISAKMP, a common protocol used for VPNs. Unless your VPN server is configured with a special option, you only get ONE VPN connection through a router.
There are a few other options that could be happening - The provider on the other end might have to put in specific firewall rules for you and doesn't want to open up the whole DHCP range that your IP gets from. They just want to add a rule for just a few static IP's assigned only to your use.
- G