selecting from either of two columns in SELECT

In SQL Server 2008R2 I have two tables, CONFIG_DATA and CNA_LIST:

CONFIG_DATA has two fields: ID [varchar(20)] and Address [int]:

01 141516

02 132132

CNA_LIST has three fields: Address [int], IP1 and IP2, both [varchar(20)]

141516 1.2.3.4 (null)

132132 (null) 2.3.4.5

These are field devices that have one or two IP addresses, in the IP1 or IP2 field (can be either, or both).

I would like to list the CONFIG_DATA table with IP1 from the CNA_LIST table, and if it's NULL, the IP2.

The Address field links the two tables.

this would work fine:

select a.ID, a.Address, b.IP1
from CONFIG_DATA a, CNA_LIST b where a.Address = b.Address

but if IP1 is null I need IP2, with this column listed as "IP":

I've tried IF(b.IP1 <> '')b.IP1 ELSE b.IP2 but the syntax checker complains.

My apologies - after 20 minutes, I still can't format this post to look right!

thanks for any help.