network issue - can't configure eth0 with .24 address as gateway

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http://ubuntuforums.org – I am configuring a server on my network. I have an IPv4 address block xxx.xxx.xxx.24/29 riding on top of a /30 block. My router has an address on the /30 block as the WAN interface, and .24/29 ( as the DMZ interface, per instructions from the network engineer, which in theory gives me .25-.29 as available addresses for servers. This configuration works fine in Windows Server. I add an ip address (xxx.xxx.xxx.25) net mask ( and gateway (xxx.xxx.xxx.24) and the network is up. In Ubuntu, when I assign the address xxx.xxx.xxx.27 and net mask, the O (Hardware)