nixpkgs/nixos/doc/manual/configuration/ipv4-config.section.md

IPv4 Configuration

By default, NixOS uses DHCP (specifically, dhcpcd) to automatically configure network interfaces. However, you can configure an interface manually as follows:

networking.interfaces.eth0.ipv4.addresses = [ {
  address = "192.168.1.2";
  prefixLength = 24;
} ];

Typically you'll also want to set a default gateway and set of name servers:

networking.defaultGateway = "192.168.1.1";
networking.nameservers = [ "8.8.8.8" ];

::: {.note} Statically configured interfaces are set up by the systemd service interface-name-cfg.service. The default gateway and name server configuration is performed by network-setup.service. :::

The host name is set using networking.hostName:

networking.hostName = "cartman";

The default host name is nixos. Set it to the empty string ("") to allow the DHCP server to provide the host name.