HP ProCurve Secure 7000dl Series Basic Management And Configuration Manual page 549

For example, an organization has allocated the address space 192.168.20.0 /24
to a remote site. However, currently the site is only using half of the addresses.
Network management have divided the network into two /25 subnets and left
the second subnet (192.168.20.128 /25) unused. You can prevent the local
router from forwarding traffic across the WAN link that will only dropped by
the remote router. Enter this command:
ProCurve(config)# ip route 192.168.20.128 /25 null 0
You could also use a null route in order to force the router to:
drop traffic to destinations that you have determined to be unauthorized
However, a better way to control traffic is to use an ACL or an ACP. (See
Chapter 5: Applying Access Control to Router Interfaces of the Advanced
Management and Configuration Guide.)
advertise a route not included in its routing table
When a router uses a routing protocol, its routing table must include a
route in order to advertise that route. You could configure a null route if
you wanted the router to advertise a route, but not to forward traffic using
that route. (For more information on this topic, see "Advertising Local
Networks" on page 13-71 in Chapter 13: IP Routing—Configuring RIP,
OSPF, BGP, and PBR of the Advanced Management and Configuration
Guide.)
IP Routing—Configuring Static Routes
Configuring Static Routes
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