The system by which daemons are launched at startup in Mac OS X, similar to init.d in Linux is explained at: http://developer.apple.com/documentation/MacOSX/Conceptual/BPSystemStartup/index.html.

to add a persistent network put your launcagent in:

/System/Library/LaunchDaemons/com.kandou.scripts.osx.addroute1.plist

content:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
"http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>Label</key>
    <string>com.kandou.scripts.osx.addroute1</string>
  <key>ProgramArguments</key>
  <array>
    <string>/sbin/route</string>
    <string>add</string>
    <string>-net</string>
    <string>192.168.154.0/24</string>
    <string>128.178.70.17</string>
  </array>
  <key>RunAtLoad</key>
    <true/>
</dict>
</plist>