Assume two IP-addresses in the same /24 network (say 1.2.3.*):
Is it safe to assume that these two hosts belong to the same AS-number? If not, roughly how often is that the case that they do not belong to the same AS?
IPv4 addresses in the same /24 can be members of different Autonomous Systems, but this is very uncommon.
I just did a quick search of the BGP routing table, and only found a couple of hundred /24s spanning different ASes. This compares to a total of ~310,000 total advertised prefixes.
Here are three examples:
If you're interested in BGP data, routeviews.org provide snapshots of the global BGP table, and a number of BGP routers accessible via telnet, for example:
hope this helps :)