When a network or group of networks is designed well, the payoff can be substantial. The payoff, however, is in hours not spent, which can be very hard to quantify. Believe me, though—designing IP space the right way, the first time, can save literally thousands of man-hours over the lifetime of the network.
IP address allocation is rarely done properly, and many unlucky network administrators end up inheriting a mess of IP networks that's just been thrown together over time. In many cases, small networks are built with no vision of where the companies might end up, resulting in massive undertakings when the IP networks need to be changed. And even the best of IP address schemes can be rent asunder by a merger or acquisition.
Think about how long it takes to put an IP address, subnet mask, and default gateway on a server. Not long at all, when you're installing the server. Now think about what is involved when the IP address, subnet mask, or default gateway needs to be changed. The server must be taken offline, which usually involves a change control. In many environments, the change needs to happen during a change-control window, which may involve you coming into the office or data center at 2:00 a.m. Now imagine that your company has 100, 200, or 1,000 servers. Don't forget that DNS and/or WINS and Active Directory will need to be updated, too.
IP network design is not a glamorous project. It is not something that the CTO will slap you on the back for in a meeting. ...