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

<section xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="ad-hoc-network-config">
 <title>Ad-Hoc Configuration</title>

 <para>
  You can use <xref linkend="opt-networking.localCommands"/> to specify shell commands to be run at the end of <literal>network-setup.service</literal>. This is useful for doing network configuration not covered by the existing NixOS modules. For instance, to statically configure an IPv6 address:
<programlisting>
<xref linkend="opt-networking.localCommands"/> =
  ''
    ip -6 addr add 2001:610:685:1::1/64 dev eth0
  '';
</programlisting>
 </para>
</section>