Solution to access net on two pcs atones

Hello,

i owned bsnl broadband conection Unlimited use .i have two pcs on ground & 1 St Floor ,my modem adsl router 1600 i s connected to my ground floor pc , My !st floor pc is also connected to modem through lan cable ,i can access internet from my G Floor PC but I cant access net from my 1 st floor PC ,Kindly suggest me the solution that i can access net on both PC simultaneously, as one pc is mine ( g Floor) & Second one is my son's (1 st floor).

< config>Windows XP / Safari 533.4</config>