In article , Nicky wrote: :Cable and dsl uses fixed numbers of MTUs? :The numbers are refering to bytes?
:The MTU concept suprises me that it can vary because i thought that :every ip packet had a fixed size and it wasnt variable.
The maximum length of an IP packet depends on the link layer protocol, and on the medium, and on any extensions supported end-to-end. "IP" is a generalized protocol that in theory has quite large maximum packets, but it is usually used over media such as twisted pair ethernet that have much lower limits (for various reasons.)
Consumer broadband cable and DSL usually use MTU that are the same as consumer ethernet, but the key to this puzzle is that there may be additional protocol layers in use on the cable or DSL line that "use up" some of the available bytes. DSL in particular often uses a format known as PPPoE, which uses up 8 bytes per packet, leaving 1492 available. Some DSL vendors use internal protocols that may use up a fair number of other bytes.
One notable layer that uses roughly 150 bytes, is "IPSec", a standardized protocol used to create Virtual Private Networks (VPNs). Cable or DSL providers might have an IPSec layer active in order to ensure the privacy of your communications -- but if they do then the maximum packet size you might have left might go down to about 1340. The end result is still 1500 bytes on the wire, but you only have access to a portion of those with the rest going for overhead.