The 8051 is irrelevant here - the RTL8091 (whatever that might be) works the ways it works irrespective of whether it's connected to an 8051, an ARM, in a PC card, or anything else!

So the real question is:and that will be answered by reading the RTL8091 Datasheet.
You should also refer to the manufacturer's site for other supporting information, such as Application Notes, examples, etc...

If you have specific questions, post a link to the document in question and describe your issue.


Ethernet is the lowest layer - the physical signalling on the wire;
IP is a higher-level protocol that can make use of ethernet;
UDP is a yet higher-level protocol that uses IP.

That's why we speak of a protocol "Stack" - because all thes layers are "stacked" up, one upon another.

Writing a protocol stack is a non-trivial exercise - you have some serious study to do here!