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Linux Mint 16 64-bit on una:

una / # grep -r "192.168.1.21" /etc/
/etc/NetworkManager/system-connections/Wired connection 1:address1=192.168.1.21/24,192.168.1.1

Relevant content of above file:
-------------------------------
[ipv4]
method=manual
dns=192.168.1.3;
address1=192.168.1.21/24,192.168.1.1
may-fail=false

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Linux Mint 17 64-bit on oksana:

# no file in /etc/NetworkManager/system-connections/

Content of /etc/network/interfaces:
-----------------------------------
# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

iface eth0 inet static
	address 192.168.1.15
	netmask 255.255.255.0
	gateway 192.168.1.1
	dns-nameservers 192.168.1.3

auto eth0
-----------------------------------
# dns-nameservers line was presumably added manually
# dns-nameservers line should immediately follow gateway line
# more than one nameserver may be listed, separated by spaces

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Linux Mint 17 64-bit on tracy:

# no file in /etc/NetworkManager/system-connections/

Content of /etc/network/interfaces:
-----------------------------------
# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

iface eth0 inet static
address 192.168.1.20
netmask 255.255.255.0
gateway 192.168.1.1
dns-nameservers 192.168.1.3

auto eth0
-----------------------------------
# dns-nameservers line was added manually
# - rest was found after setting Static IP
# using Control Panel -> Network

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