デフォルトのネットマスク

/etc/netmasksで設定するらしい

$ cat /etc/netmasks
#
# The netmasks file associates Internet Protocol (IP) address
# masks with IP network numbers.
#
# network-number netmask
#
# The term network-number refers to a number obtained from the Internet Network
# Information Center.
#
# Both the network-number and the netmasks are specified in
# "decimal dot" notation, e.g:
#
# 128.32.0.0 255.255.255.0
#
172.29.64.0 255.255.255.0
10.177.129.0 255.255.255.0
10.177.132.0 255.255.255.0