/etc/ypserv.securenets is in nis 3.17-34ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | #
# securenets	This file defines the access rights to your NIS server
#		for NIS clients (and slave servers - ypxfrd uses this
#		file too). This file contains netmask/network pairs.
#		A clients IP address needs to match with at least one
#		of those.
#
#		One can use the word "host" instead of a netmask of
#		255.255.255.255. Only IP addresses are allowed in this
#		file, not hostnames.
#
# Always allow access for localhost
255.0.0.0	127.0.0.0
# This line gives access to everybody. PLEASE ADJUST!
0.0.0.0		0.0.0.0
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