nixpkgs/nixos/doc/manual/configuration/ad-hoc-network-config.section.md

Ad-Hoc Configuration

You can use to specify shell commands to be run at the end of network-setup.service. This is useful for doing network configuration not covered by the existing NixOS modules. For instance, to statically configure an IPv6 address:

networking.localCommands =
  ''
    ip -6 addr add 2001:610:685:1::1/64 dev eth0
  '';