Solving IP Endianness in x64 Assembly: A Single-Pass Algorithm

Research Context When doing low-level network programming in Assembly, you experience firsthand the immense chaos running behind the scenes of operations we solve with a single line in high-level languages (Python, C, etc.). While developing the Nested-ICMP-Communication Analysis project, specifically an Encapsulated ICMP framework, I hit exactly this kind of wall: extracting an IP address from a packet header and printing it to the screen in the correct format. Sounds simple, right? However, when x86 architecture and network protocols are involved, seeing 5.1.168.192 instead of 192.168.1.5 on your terminal is extremely common. So why does this happen, and what kind of algorithm did I develop to overcome this issue during the debugging process? Let's dive into the background. The Endianness Problem in Network Headers When you capture a packet coming over the network and read the source/destination IP address inside the sockaddr_in structure, the data arrives in Network Byte Order (Big-Endian) format. This means the most significant byte is stored at the lowest memory address. However, the x86/x64 processor architectures we use rely on Little-Endian (Host Byte Order). When the processor pulls this 4-byte IP data into a register, the reading direction is effectively reversed for our purposes. The result? A packet that arrives as 192.168.1.5 appears scrambled if we try to naively print it from memory. The inet_ntoa() function in high-level languages handles this conversion in the background. But if you are writing a custom sniffer in pure Assembly, you must do this conversion byte by byte yourself. Debugging Hell: The Problems Encountered While writing this conversion, I encountered a few critical issues that cost me hours in GDB (GNU Debugger): Register Clashes: While separating each octet (byte) of the IP address and converting it to an ASCII character (string), you must use the AX register for division operations (DIV). If you don't carefully manage your remainders