Hi Rootstock, You're from China!! Thanks. The reason I ask for because I saw the link below which confusing me http://rute.sourceforge.net/node28.html Table 25.1 20-IHL*4-1 Options + padding to round up to four bytes Any idea? anyong ""xie rootstock"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > IP and TCP header has 20bytes Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=13706&t=13706 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]