IPv4 vs IPv6: differences and dual-stack configuration on Linux

    What is the difference between IPv4 and IPv6?

    IPv4 and IPv6 are two versions of the Internet Protocol used to identify devices on a network.

    Feature IPv4 IPv6
    Address length 32 bits 128 bits
    Address format Dotted decimal: 203.0.113.10 Hexadecimal colon: 2001:0db8:85a3::8a2e:0370:7334
    Total addresses ~4.3 billion ~340 undecillion (3.4 × 10³⁸)
    Configuration Manual or DHCP SLAAC (auto), DHCPv6, or manual
    NAT Widely used (required due to address scarcity) Not needed (every device gets a public address)
    Header size Variable (20–60 bytes) Fixed (40 bytes, simpler)
    IPSec Optional Built-in (mandatory in specification)
    Broadcast Yes No (uses multicast instead)

    IPv6 address types

    Type Prefix Description
    Global Unicast 2000::/3 Public, routable addresses (equivalent to public IPv4)
    Link-Local fe80::/10 Auto-assigned, only valid within the local network segment
    Unique Local (ULA) fd00::/8 Private addresses (equivalent to 10.x.x.x, 192.168.x.x in IPv4)
    Loopback ::1 Equivalent to 127.0.0.1

    Check your current IP configuration

    View both IPv4 and IPv6 addresses

    ip addr show

    View only IPv4

    ip -4 addr show

    View only IPv6

    ip -6 addr show

    Check default routes

    ip -4 route show    # IPv4 routes
ip -6 route show    # IPv6 routes

    Dual-stack configuration

    Dual-stack means running IPv4 and IPv6 simultaneously on the same interface. This is the recommended approach for transitioning to IPv6 while maintaining IPv4 compatibility.

    Netplan (Ubuntu 18.04+)

    network:
  version: 2
  ethernets:
    ens18:
      addresses:
        - 203.0.113.10/24
        - 2001:db8::10/64
      routes:
        - to: default
          via: 203.0.113.1
        - to: default
          via: 2001:db8::1
      nameservers:
        addresses:
          - 8.8.8.8
          - 2001:4860:4860::8888

    Apply with:

    sudo netplan apply

    /etc/network/interfaces (Debian)

    auto eth0
iface eth0 inet static
    address 203.0.113.10
    netmask 255.255.255.0
    gateway 203.0.113.1

iface eth0 inet6 static
    address 2001:db8::10
    netmask 64
    gateway 2001:db8::1

    Apply with:

    sudo systemctl restart networking

    NetworkManager (AlmaLinux / RHEL)

    sudo nmcli connection modify "System eth0" \
  ipv4.addresses 203.0.113.10/24 \
  ipv4.gateway 203.0.113.1 \
  ipv4.method manual \
  ipv6.addresses 2001:db8::10/64 \
  ipv6.gateway 2001:db8::1 \
  ipv6.method manual

    sudo nmcli connection up "System eth0"

    Testing IPv6 connectivity

    ping -6 ipv6.google.com
ping6 ipv6.google.com     # older syntax
traceroute -6 ipv6.google.com
curl -6 https://ipv6.google.com

    Disabling IPv6 (if needed)

    In some cases, you may need to disable IPv6 temporarily:

    sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1
sudo sysctl -w net.ipv6.conf.default.disable_ipv6=1

    To make it persistent, add to /etc/sysctl.conf:

    net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

    Then apply: sudo sysctl -p

    Firewall considerations

    If you use UFW, it handles IPv4 and IPv6 automatically (if IPv6 is enabled in /etc/default/ufw). Your rules apply to both protocols:

    sudo ufw allow ssh    # Allows SSH on both IPv4 and IPv6

    If you use iptables directly, note that IPv6 rules require ip6tables:

    sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT     # IPv4
sudo ip6tables -A INPUT -p tcp --dport 22 -j ACCEPT    # IPv6
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