FreeBSD Alias another Ip BLOCK

I have a first ip block on a dif subnet with the netmask 255.255.255.240 added
now i had another on a seperate subnet with the netmask 255.255.255.240 also, freebsd wont let me use 255.255.255.240 as an alias'd netmask..so what is the correct way to alias these ips

 

 

 

 

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