Ran into an interesting project while deploying a couple of sites using Comcast Fiber Internet and Sonicwall firewalls. Most providers like AT&T usually give all usable IPs in 1 block, Comcast breaks this up into a routing block (/30) and then usable IPs (/29) separate. In this scenario you essentially setup the /30 like you normally would on the Sonicwall WAN interface and then use static ARP to handle the /29 block.
Currently Sonicwall does not allow assigning multiple IPs to the primary/secondary WAN interface. The “secret sauce” to making this work is to add an ARP address on your WAN interface for each of the /29 addresses you want to use. In short these are the steps:
1) Setup /30 address on WAN interface as usual
2) Create the /29 range as an address object/s
3) Add an ARP address on WAN interface for each of the /29 addresses you want to use
4) Create a routing policy to route the /29 addresses outbound of the WAN interface
5) Create usual NAT/firewall/address objects/rules for the /29 addresses
A more detailed walk-through here: https://support.software.dell.com/kb/sw3726