An interesting question was asked to me and I did not know what to answer.. So I'll ask here.

Let's say I subscribed to an ISP and I'm using cable internet access. ISP gives me a public IP address of 60.61.62.63.

What keeps me from changing this IP address to, let's say, 60.61.62.75 and mess with another consumer's internet access?

For the sake of this argument, let's say that this other IP address is also owned by the same ISP. Also, let's assume that it's possible for me to go into the cable modem settings and manually change the IP address.

Under a business contract where you are allocated static addresses, you are also assigned a default gaetway, a network address and a broadcast address. So that's 3 addresses the ISP "loses" to you. That seems very wastefull for dynamically assigned IP addresses where the majority of customers are..

Could they simply be using static arps? ACLs? Other simple mechanisms?

Anyone who worked at an ISP would be willing to explain this a bit?