c語言裡的0L是什麼意思?

就是
long int 0
你的程式可能是在比對 ETHNET的封包內容的IP ADDRRESS
所以IPADDR的宣告應該也是 Long int

Do this & you will find the answers all by yourself:

printf("sizeof(0L) is %d", sizeof(0L));

if you run 64bit OS on 64bit processors, you may get an answer of 8 octects.The only suggestions given by K&R is - long is not shorter than int while int is not shorter than short.

就是0 只是這個0是long型態的

我來翻譯一下bean_boom_2大的回答：
0L在C語言是以長整數(至少32位元容量)儲存(表示)的0。依照版主所貼的程式碼，似乎是表示一個ip位址，那麼應該就是32位元。

0L in C means zero in long (32 bit at least). It seems the code you have posted is for an IP address and thus it should be 32 bit.