What is IPv6
IPv6 (Internet Protocol Version 6) is the second-generation network-layer protocol standard, designed to replace IPv4. Its most obvious difference from IPv4 is address length: an IPv6 address is 128 bits long, written in colon-separated hexadecimal notation (for example, 2001:db8::1/64), compared to IPv4's 32 bits — solving the address-exhaustion problem that IPv4 has run into as the Internet has grown.
But IPv6 isn't just a bigger address space bolted onto the old protocol. It replaces core IPv4 mechanisms — ARP and ICMP router discovery among them — with a redesigned, more capable set of protocols built around neighbor discovery and address autoconfiguration, changing how devices find each other and get configured on a network.
How IPv6 Works
A device interface running IPv6 can hold several kinds of address: a global unicast address (routable across the Internet), a unique local address (routable only within a private network), or a link-local address. Link-local addresses (from the FE80::/10 block) are special — they're generated automatically the moment IPv6 is enabled on an interface, and they're commonly used for neighbor discovery and for establishing routing-protocol neighbor relationships, without needing any address configuration at all.
Most of what makes IPv6 distinct from IPv4 lives in the Neighbor Discovery Protocol (NDP), which takes over the jobs ARP and ICMP router discovery handled in IPv4:
Address resolution: devices on the same link exchange Neighbor Solicitation (NS) and Neighbor Advertisement (NA) messages to learn each other's link-layer addresses, maintaining a neighbor cache table — IPv6's equivalent of an ARP table.
Router discovery: hosts send Router Solicitation (RS) messages and receive Router Advertisement (RA) messages in response, learning what routers exist on the link and how the network is configured.
Address autoconfiguration: hosts can either use the prefix information carried in RA messages to self-assign an address through Stateless Address Autoconfiguration (SLAAC), or fall back to DHCPv6 for stateful configuration when more explicit control is needed.
Router Advertisement (RA) is where a device acting as an IPv6 router tells hosts on the link how to configure themselves. Beyond the address prefix used for SLAAC, an RA message carries a set of flags that shape host behavior: the M flag (managed) tells hosts whether to get their address via DHCPv6 instead of SLAAC, the O flag (other) tells hosts whether to get other configuration — like DNS — via DHCPv6, and the A flag (autonomous) controls whether a given prefix is usable for SLAAC at all. RA messages can also carry routing information (specific routes and their preference) and other parameters like DNS server addresses and link MTU — giving a router fairly complete control over how hosts on the link get configured, all without a DHCP server in the picture if SLAAC alone is sufficient.
Why IPv6 is Beneficial
Solves address exhaustion at the source: A 128-bit address space is large enough that address scarcity — the problem driving techniques like CGNAT under IPv4 — simply isn't a concern under IPv6.
Configures itself with SLAAC: Hosts can obtain a usable address purely from RA messages, without needing a DHCP server, simplifying deployment in networks where full stateful control isn't necessary.
Still supports centralized control when needed: The M and O flags let a network fall back to DHCPv6 for address assignment or ancillary configuration exactly where more control matters, rather than forcing an all-or-nothing choice between SLAAC and DHCP.
Neighbor discovery built for the modern link: NDP's NS/NA and RS/RA exchanges replace ARP and ICMP router discovery with a protocol designed specifically for IPv6's addressing model, rather than adapting IPv4-era mechanisms.
Link-local addressing without configuration: Automatically generated link-local addresses mean neighbor discovery and routing-protocol adjacencies can be established immediately, without any manual address planning for that purpose.