Ethernet Communication with Microcontroller
AT89C2051 doesn't have SPI peripheral to communicate with the enc28j60. You would have to bit bang SPI (software SPI).
The enc28j60 doesn't handle the TCP/IP stack, so the MCU must handle it, but the AT89C2051 has only 2kB of memory.
The 8 kB enc28j60 internal memory can be used to store network packages, but to handle this over bit banged SPI really doesn't make much sense.