VulnTech How PCs get IPv6 addresses: NDP and SLAAC | CCNA Exam Prep

Neighbor Discovery Protocol (NDP)

IPv6 has no ARP. Its job, and more, is done by NDP, five ICMPv6 messages:

Message	Sent to	Job
Router Solicitation (RS)	ff02::2 (all routers)	A PC asks: "any routers here?"
Router Advertisement (RA)	ff02::1 (all nodes) or the PC	A router answers: "I'm here, the prefix is 2001:db8:acad:10::/64, use me as gateway"
Neighbor Solicitation (NS)	the solicited-node group	"Who has 2001:db8:acad:10::1? Tell me your MAC" (like an ARP request)
Neighbor Advertisement (NA)	the asker	"That's me, here's my MAC" (like an ARP reply)
Redirect	the sender	"Use another router for that destination"

SLAAC: Stateless Address Autoconfiguration

The PC builds its link-local address by itself (fe80:: + an interface ID).
It sends an RS. The router answers with an RA holding the /64 prefix.
The PC adds its own interface ID to the prefix: that's its global address. Linux uses EUI-64 from its MAC; Windows and macOS use a random interface ID for privacy.
DAD (Duplicate Address Detection): before using the address, the PC sends an NS for it. If nobody answers, it's unique.
The PC's default gateway is the router's link-local address, taken from the RA.

No server keeps track of who has which address: that's why it's "stateless". Here are both PCs after SLAAC:

In NetLab

▶ PC1
NetLab Windows-style command prompt. Type help to see the commands.
C:\Users\student>ipconfig

Windows IP Configuration


Ethernet adapter Ethernet:

   Connection-specific DNS Suffix  . : 
   IPv6 Address. . . . . . . . . . . : 2001:db8:acad:10:11db:99de:fbce:36d1
   Link-local IPv6 Address . . . . . : fe80::11db:99de:fbce:36d1%12
   Default Gateway . . . . . . . . . : fe80::1%12
▶ PC2
student@PC2:~$ ip -6 addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    inet6 ::1/128 scope host noprefixroute 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    inet6 2001:db8:acad:20:21a:f2ff:fe3a:a4ee/64 scope global dynamic mngtmpaddr 
       valid_lft 2591998sec preferred_lft 604798sec
    inet6 fe80::21a:f2ff:fe3a:a4ee/64 scope link 
       valid_lft forever preferred_lft forever
student@PC2:~$ ip -6 route
2001:db8:acad:20::/64 dev eth0 proto ra metric 100 pref medium
fe80::/64 dev eth0 proto kernel metric 256 pref medium
default via fe80::1 dev eth0 proto ra metric 1024 pref medium

PC1 (Windows) has a random-looking interface ID; PC2 (Linux) has 21a:...:ff:fe.., which is EUI-64 from its MAC. Both use fe80::1, R1's link-local address, as the gateway.

The three ways to give hosts IPv6 settings

Method	Address from	DNS server from
SLAAC	RA prefix + own interface ID	RA (RDNSS option), if the router sends it
Stateless DHCPv6	SLAAC	A DHCPv6 server (RA has the O flag set)
Stateful DHCPv6	A DHCPv6 server, which tracks leases	The DHCPv6 server (RA has the M flag set)

The router's neighbour table is IPv6's ARP table:

In NetLab

▶ PC1
NetLab Windows-style command prompt. Type help to see the commands.
C:\Users\student>ping 2001:db8:acad:20::1

Pinging 2001:db8:acad:20::1 with 32 bytes of data:
Reply from 2001:db8:acad:20::1: time<1ms
Reply from 2001:db8:acad:20::1: time<1ms
Reply from 2001:db8:acad:20::1: time<1ms
Reply from 2001:db8:acad:20::1: time<1ms

Ping statistics for 2001:db8:acad:20::1:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms
▶ R1
R1>enable
R1#show ipv6 neighbors
IPv6 Address                              Age Link-layer Addr State Interface
2001:DB8:ACAD:10:11DB:99DE:FBCE:36D1        0 001a.33d5.62b1 REACH Gi0/0

Remember: RA = prefix + gateway. SLAAC = the PC builds its own address from the RA. The gateway is the router's link-local address. All of it needs ipv6 unicast-routing on the router.