First... you really shouldn't be running your wireless open, especially if unencrypted. Your data transmission may not be secure, and I would certainly hope you do not do your banking from that connection. If I were one of your neighbors you may already be in trouble.
Second, even if your internet is *really* fast, your wireless network could get slowed down depending on the quality of your wireless router. If a couple of your neighbors are running torrents and a couple more are avidly gaming, your WAP could easily get overworked.
Finally you might want to check your ISP's ToS, more and more with the broader ranger of wireless routers, ISP's are packing in anti-connection sharing clauses into their agreements in order to keep 1 household from being a pseudo ISP for a block or small community.
Now on to your actual question "I get destination host unreachable". -- First, your routers DHCP should list all devices directly connection to your WAP... however, there is a fair chance that connecting devices my not be PC's, it could be handhelds, a Nintendo Wii, another wireless router being used to serve ethernet connected PC's in which case your neighbor could be connecting an entire household of devices behind a NAT... there are a number of reasons you get unreachable.
or... it could also be your programming. Why don't you pick up a program called "Angry IP Scanner" http://www.angryip.org/w/Home
and see if that returns any different results. It's also open source, so you might be able to learn a few things from that as well.