ODD NUMBER IP ARE BLOCKED TO INTERNET ACCESS

hi everybody we have 50 no of workstation .DHCP is configured win2003 server(scope is10.64.0.1-10.64.1.100 ),
Internet going through a fortigate 200A firewall, no restriction for outbound connection , So now we found all odd no ip eg. 61,63,65,…71,73,75…79…81..93.. are blocked .. dhcp is assigning these ip but in tose system lan is working but internet is not working can ne one give ne clue whta is going on our network..