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FreeBSD: IPv6 Configuration Set Prefixlen Value

How do I set the prefixlen for an IPv6 address under FreeBSD operating systems?

You can set the prefixlen for an IPv6 address by editing /etc/rc.conf file, enter:
# vi /etc/rc.conf
Assuming that your IPv6 is address is 2001:48c8:7::2 for em0, enter:

ipv6_ifconfig_em0="2001:48c8:7::2 prefix 64"

Save and close the file. Restart networking. You can also set it at the shell prompt using the ifconfig command as follows:

ifconfig interFace inet6 ipv6IpAddress prefixlen Value
ifconfig fxp0 inet6 2001:48c8:7::2 prefixlen 64


  • fxp0 or em0 - FreeBSD NIC interface name
  • 2001:48c8:7::2 - IPv6 public IP address
  • 64 - IPv6 Prefixlen Value