If you are setting up VNET-to-VNET connectivity between two virtual networks using http://msdn.microsoft.com/en-us/library/azure/dn690122.aspx, here is the summary of the process:
Once the above process is complete, the network connectivity is established between VNET1 and VNET2. if you are doing this first time the documentation tells you (steps 3 and 4) to use a place holder IP address for the yet to be created gateway.
This placeholder can’t be any IP address from the private IP address spaces. Since this placeholder will be replaced with the gateway address created during steps 7 and 8, use anything (e.g. 25.345.131.1) that resembles a public IP address.
Note that the address spaces of VNET1 and VNET2 are mutually exclusive as these two VNETs will be combined into one logical address space after the connectivity is established.
Hope this helps to save a few minutes if you are doing this first time.