i want to know ,when i made download by program like (internet download manger)

which port it could connect to make download??

and if i want to limit only download with out browsing could i make that by limiting connection in firewall by using this port?!

It's an HTTP transaction just like looking at this forum is. You know know something is large file download until as many bytes as you think the threshold should be have passed through the connection. Search the forum for "connection-bytes".