How is your home network set up? Do you use local DHCP without reservations, so that your Mac potentially gets a new IP address every time its lease runs out? Are you on WiFi or network cable to connect to your router?

How do you connect to your ISP? Do you have DSL, which reconnects the PPP tunnel once every 24 hours? Are you using a router that does NAT? Or does your machine connect directly to the internet?

Are you using any kind of VPN? Remember that services like WTFast or similar "game network speed enhancers" basically use VPN in order to try lowering their customers' network latency.

The method ZOS uses is called IP authorization, and is part of a two factor authentication (the first factor are your credentials). The login server registers the IP address range the client is in. ISPs can have many different address ranges, and since IP addresses are usually assigned dynamically to customers from a pool, the login server needs to verify each and every new range the client tries to connect with. For me (DTAG DSL), this happened only twice in the past year, so factors like a whois lookup or geolocation might play a role, too.

If you're getting a new authorization request every time your Mac starts up, there must be a reason why the login server thinks that the IP address range your client is in is a new one. Using a VPN on the Mac would be a likely explanation, but I'm curious about your setup.