Question: How do I concatenate two binaries in Erlang?

For example, let's say I have:

```
B1 = <<1,2>>.
B2 = <<3,4>>.

```

How do I concatenate B1 and B2 to create a binary B3 which is <<1,2,3,4>>?

The reason I am asking this is because I am writing code to encode a packet for some networking protocol. I am implementing this by writing encoders for the fields in the packet and I need to concatenate those fields to build up the whole packet.

Maybe I am doing this the wrong way. Should I build up the packet as a list of integers and convert the list to a binary at the last moment?

Answer: 
```
28> B1= <<1,2>>.
<<1,2>>
29> B2= <<3,4>>.
<<3,4>>
30> B3= <<B1/binary, B2/binary>>.
<<1,2,3,4>>
31>

```
To use an io\_list, you could do:

```
erlang:iolist_to_binary([<<"foo">>, <<"bar">>])

```

Which is nice and legible. You could also use lists and things in there if it's more convenient.