Wildcards Mini-Tutorial

        OFMatch match = new OFMatch()

        match.setNetworkSource(IPv4.toIPv4Address("192.168.12.0"));      // (1)
        match.setNetworkDestination(IPv4.toIPv4Address("10.0.0.0"));       // (2)
        match.setWildcards(Wildcards.FULL.withNwSrcMask(24).withNwDstMask(8));      // (3) 

Explanation: Setting an IP address with a netmask in an OFMatch is a two step process. You set the actual IP address with setNetworkSource or setNetworkDestination (steps (1) and (2) ). Then you set the appropriate wildcard bits on the match so only parts of your IP addresses are being matched. 

What are the wildcard bits?
OpenFlow 1.0 uses a 32-bit wildcards field that has binary flags for most fields in the match. E.g., you can decide to match on or wildcard dl_src, dl_dst, dl_type, in_port etc. Network Source and Destination addresses are special however: they are the only fields that can be partially wildcarded. This requires some obnoxious bitshifting to convert from the usual CIDR style netmask (/8) to the appropriate OpenFlow wildcards representation. The details are in the spec, and highly confusing :) 

What's an easier way to do it?
Floodlight has a convenience API called Wildcards that allows you to construct a correct Wildcard bitmask more easily.
The way you use it is you start out with either Wildcards.FULL (all bits wildcarded) or Wildcards.EXACT (no bits wildcarded). Then you set / unset the appropriate Wildcard bits using wildcard(Flag) or matchOn(Flag). The Layer 3 Network Source Masks and Network Destination Masks are set with 'withNwSrcMask' or withNwDstMask. These methods take normal CIDR-style netmask lengths and do the conversion for you.

Example for a match that matches on in_port, dl_type, and on IP src/dst masks:

match.setWildcards(Wildcards.FULL.matchOn(Flag.IN_PORT).matchOn(Flag.DL_TYPE).withNwSrcMask(24).withNwDstMask(8));

Example of how to modify the wildcards:

match.setWildcards(Wildcards.FULL.matchOn(Flag.IN_PORT).matchOn(Flag.DL_TYPE).withNwSrcMask(24).withNwDstMask(8));

What doesn't work?

Most of the other stuff that was suggested in this thread was somehow wrong. In particular.

((Integer)sw.getAttribute(IOFSwitch.PROP_FASTWILDCARDS)).intValue() queries a generic switch property and has nothing to do with setting wildcards flags on flow mods.

int MY_SHIFT_BITS = 16;
int my_nw_src_mask = ((1 << OFMatch.OFPFW_NW_SRC_BITS) - 1) << MY_SHIFT_BITS;   this is the wrong way to manually compute the wildcard masks