Henrik, i wanted to comment to your original post, so I tried to restart a new ref to original, but to add (comment), ran into max chars so I deleted some letters to make room for my comment. But if it would be less confusing, a new named thread could be done.
Checking a little further, your using the 5100 chip and working closer with its coding compared to the modules where there is less or no directly accessing the networking controller chip, aside from the few basic tcp/ip and serial settings.
If you havn't yet, try W3schools.com. Covers about everything and free.

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The Wiz701 serial to ethernet module I use stores it settings in flash reguarding power-up.

Code:
'''-------TX INT ROUTINE-------------AX is start adderss in prog flash------''''
TRint:
READCODE AX , B4         'SEND OUT FLASH MEM
if b4=0 then 
@ INT_DISABLE TX_INT   
else
txreg=b4       
AX=AX+1                         
endif
@ INT_RETURN
I send with TX interrupt , like you said, readcode and check for 0 to stop transmit. Allows base PIC program to keep going even with send 1-2000 chars

Don