The World Where IPv6 Was a Good Design (2017)

Networks originally started on point-to-point circuits. Back then, only the devices at both ends were needed, and addresses were barely necessary.

Then the LAN appeared, and things changed. Once multiple devices were connected onto a single bus, a layer 2 address was needed to distinguish each device, and eventually the Ethernet MAC address became the de facto standard. Because MACs are assigned sequentially at the time of factory shipment, it's hard to organize routes hierarchically, and in large-scale networks this led to problems like bridge tables, flooding, and loops.

The internet introduced IP routing to create connections beyond the LAN. But to use IP on top of a LAN, you had to choose, separately from the final destination IP, the next-hop router within the same segment that would receive the packet. So while it looks in practice like you're "specifying a router by IP address," internally you first have to figure out that router's MAC address.

ARP was the mechanism that took on this role. ARP broadcasts across the entire local network to find the device that has a given IP. The problem is that this broadcast easily balloons in bridged and wireless environments, becoming a heavy burden at large scale. Various equipment adopted all sorts of workarounds to reduce ARP traffic, but the key point is that this itself was already a sign of complexity.

After that, most networks effectively settled into an Ethernet bus + IP router structure, and automatic IP address assignment became necessary. But since IPv4 is 32 bits, it couldn't be assigned endlessly in a factory-like fashion the way MACs are. That's why BOOTP/DHCP came about, but these too are essentially just special protocols for borrowing an IP, and in the end you still have to maintain a separate step linking the Ethernet address and the IP address.

The article's conclusion is clear. The reason IPv6 looks complicated isn't simply that it was badly designed—it's because it tried to layer a new addressing scheme on top of already-entrenched legacy systems like Ethernet/MAC/ARP/DHCP. Just "adding more bits to IPv4" couldn't solve these historical constraints, and so, beyond a larger address space, IPv6 had to redesign a number of other design elements as well.