The IP packet is the layer 3 packet. When talking about MTU (not ip mtu) means the particular interface maximum layer2 payload. If we are talking about Ethernet where the MTU is 1500 (not talking about baby jumbo or jumbo frames) meaning the ethernet payload is 1500 bytes so the longest IP packet (including header) cannot be longer than 1500 bytes.

This 1500 byte purely the payload and does not include the header and CRC of tzhe layer2 frame.

Hope it clears,


carl_townshend Wed, 09/05/2007 - 03:53
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so when I send some infomation to another pc, depending on how much data, will it always use 1500 bytes per packet ?


It means only that the Ethernet frame will not be longer than 1500 + header+FCS. If the IP packet is longer than 1500 it will be fragmented.

It depends on the application and transport protocol how large packets will be sent.

The large volume of data does not necessarily mean large packets/frames. Just think about ATM where fixed 53 bytes cells are sent over the link.

If you have small packets (like voip traffic) the ethernet frame will be short as well. However as I know there is a minimum size for ethernet frame is 64 bytes (including header and FCS).

Hope it helps,


Joseph W. Doherty Wed, 09/05/2007 - 04:03
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"ip packets, what are they? "

They are the construct used by IP for sending information. Analogous to a mail package/envelope/packet. Each has addressing information in it, sender's and intended recipient's.

"are they a specific size"

No, they vary in size. There is a minimum and a maximum, although the latter is often first limited by the physical medium being used.

"is the packet in the Ethernet frame ?"

When using IP on Ethernet, yes. (Ethernet frame doesn't have to have IP in it.)

carl_townshend Wed, 09/05/2007 - 08:27
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so does the ethernet frame get changed when crossing ppp links, serial, frame relay links etc ?

Joseph W. Doherty Wed, 09/05/2007 - 09:03
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As the IP packet moves across different physical media, it's "repackaged" into what that physical media supports. It could be a new Ethernet frame or something else.

Although frames and packets are often lumped together, they are different but serve similar purposes.

carl_townshend Thu, 09/06/2007 - 08:38
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so would we have 1 packet per ethernet frame? or can we have many packets in a frame ?

Joseph W. Doherty Thu, 09/06/2007 - 10:43
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Normally, would expect just 1 IP packet per 1 Ethernet frame. (The reason I write normally, in theory I could see putting multiple small IP packets in a frame provided there was some information to delimit them. Don't know of anything doing so though.)


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