When examining IPv6 address assignment in Ubuntu, many administrators notice an interesting behavior: the system receives both a SLAAC-configured address with /64 prefix (from Router Advertisements) and a DHCPv6-assigned address with /128 prefix. This raises valid questions about network design principles.

In your example output:

inet6 2001:xxxx:1f15:yyyy::9/128 scope global
inet6 2001:xxxx:1f15:yyyy:faa9:63ff:fe0c:3ccb/64 scope global dynamic

We see two distinct address assignment mechanisms at work:

• The /64 address comes from SLAAC (Stateless Address Autoconfiguration) using the interface identifier
• The /128 address comes from stateful DHCPv6 assignment

DHCPv6 typically assigns /128 addresses for several important reasons:

1. Address Exclusivity: A /128 prefix guarantees no other host will receive this exact address
2. Lease Management: DHCP servers need precise control over individual addresses for lease tracking
3. RFC Compliance: RFC 8415 doesn't mandate prefix delegation for non-PD cases

Here's a typical ISC DHCPv6 server configuration that would produce this behavior:

subnet6 2001:db8:1f15::/64 {
    range6 2001:db8:1f15::100 2001:db8:1f15::200;
    prefix6 2001:db8:1f15:: 2001:db8:1f15:: /64;
}

Contrast this with a prefix delegation setup where you might see different behavior:

subnet6 2001:db8:1f15::/64 {
    prefix6 2001:db8:1f15:: /64;
    allow leasequery;
}

The /128 assignment affects several operational aspects:

• Routing: Hosts need proper RA configurations to ensure routing works correctly
• Firewalling: Security policies must account for both address types
• Monitoring: Network monitoring tools should track both SLAAC and DHCPv6 addresses

To check how your Ubuntu system obtains its IPv6 addresses:

# Check DHCPv6 client process
ps aux | grep dhclient

# View DHCPv6 leases
cat /var/lib/dhcp/dhclient6.leases

# Verify networkd configuration (if using systemd-networkd)
cat /etc/systemd/network/*.network

You might see /64 assignments from DHCPv6 in these scenarios:

• When using prefix delegation (PD)
• With certain enterprise-grade DHCP servers
• When explicitly configured for prefix assignment

When testing IPv6 implementation in Ubuntu, you might notice that while Stateless Address Autoconfiguration (SLAAC) via Router Advertisements (RA) correctly assigns a /64 address, DHCPv6 often assigns an additional IPv6 address with a /128 prefix. Here's a typical interface configuration:

2: eth0:  mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether f8:a9:63:0c:3c:cb brd ff:ff:ff:ff:ff:ff
    inet6 2001:xxxx:1f15:yyyy::9/128 scope global 
       valid_lft forever preferred_lft forever
    inet6 2001:xxxx:1f15:yyyy:faa9:63ff:fe0c:3ccb/64 scope global dynamic 
       valid_lft 82162sec preferred_lft 10162sec
    inet6 fe80::faa9:63ff:fe0c:3ccb/64 scope link 
       valid_lft forever preferred_lft forever

The /128 prefix length for DHCPv6-assigned addresses is actually standard behavior per RFC 3315. Unlike IPv4 where DHCP typically provides both address and subnet mask, DHCPv6 assigns individual host addresses with an implicit /128 prefix because:

• IPv6 doesn't use broadcast or ARP, making traditional subnet masks unnecessary
• The address is treated as a standalone endpoint identifier
• Routing information comes separately from Router Advertisements

While the /128 prefix is normal, your DHCPv6 server configuration can influence this behavior. For example, in ISC DHCPv6 server (dhcpd6.conf), you might see:

subnet6 2001:db8:1f15::/64 {
    range6 2001:db8:1f15::100 2001:db8:1f15::200;
    # Prefix delegation example (different from address assignment)
    prefix6 2001:db8:1f00::/56 2001:db8:1f00::/60;
}

The range6 directive specifies individual host addresses that will be assigned with /128 prefixes, while prefix6 is used for prefix delegation (typically /56 or /60 for downstream networks).

This behavior has several practical consequences:

1. Address Scope: The /128 address is globally routable but treated as a single host
2. Duplicate Address Detection: Still occurs for DHCPv6-assigned addresses despite the /128 prefix
3. Firewall Rules: You'll need to account for both SLAAC (/64) and DHCPv6 (/128) addresses in your rules

Here's an example nftables rule that handles both address types:

table ip6 filter {
    chain input {
        type filter hook input priority 0;
        ip6 saddr 2001:db8:1f15::/64 accept
        ip6 saddr 2001:db8:1f15::9/128 accept
    }
}

If you're seeing unexpected behavior with DHCPv6 addresses:

• Check radvd and DHCPv6 server configurations for consistency
• Verify that sysctl net.ipv6.conf.eth0.accept_ra is set appropriately (usually 2)
• Inspect DHCPv6 client logs with journalctl -u systemd-networkd or dhclient -6 -d eth0

Remember that Ubuntu's default network manager (Netplan) uses systemd-networkd or NetworkManager as the backend, each with slightly different DHCPv6 implementations.