how to descripe partly bus connection between two module

module a(
......
output reg [11:0] a
);

module b(
.......
input wire [3:0] b
);

module c(
.....
input wire [7:0] c
);

Now I need to connect Module a's output a[3:0] to Module b's input b,
and
Module a's output a[11:4] to Module
c's input c
how can I descripe it ?

thanks in advance.