after moving to a new place i had to setup my home network. when i connected my first laptop to the comcast modem , everything was "hunky-dory". so , just before i connected the wireless router i tried to directly connect the other laptop. this lead to "limited or unavailable network connectivity".

i tried "ipconfig /all" on the XP cmd window. this responded with a strange looking 169 ip address.

being a techy i tried to ping this ip - it responded immediately with <1ms round trip time. so obviously there was a server locally , it wasn't going anywhere in the network.

opened the network connections and looked at the tcp/ip properties for the connection.on the "alternate configuration" found :"automatic private ip".

so- when DHCP is not available it goes to this "automatic private ip" that assigns the address 169.***

this lead me to believe that the modem(cable ->comcast in my case) caches entries of the computer it's just assigned an ip to!

the solution:
*disconnect power from modem
*disconnect power from router(if any)
*connect modem ( after at least 1 min of wait time)
*connect router ( a little after ,say another minute)
*connect computer

this will always solve the problem.
hope this helps.