Here is my /etc/network/interfaces:

**Note: eth0 and eth2 are for virtualbox virtual nics to bridge to so I just commented it out and left them alone

# The loopback network interface
auto lo
iface lo inet loopback

## eth0 TPLink Realtek
# auto eth0
# iface eth0 inet static
# address 10.10.10.10
# netmask 255.0.0.0

# eth1 mobo intel nic
auto eth1
iface eth1 inet static
address 61.xx.yy.zz
netmask 255.255.255.0
gateway 61.xx.yy.254
dns-nameservers 8.8.8.8

## eth2 intel nic
# auto eth2
# iface eth2 inet static
# address 10.10.20.20
# netmask 255.0.0.0

and my ifconfig:

eth0      Link encap:Ethernet  HWaddr a0:f3:c1:00:52:50  
          inet6 addr: fe80::a2f3:c1ff:fe00:5250/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:131 errors:0 dropped:0 overruns:0 frame:0
          TX packets:57 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:13885 (13.5 KiB)  TX bytes:14304 (13.9 KiB)
          Interrupt:42 Base address:0x4000 

eth1      Link encap:Ethernet  HWaddr 30:85:a9:a7:a0:bb  
          inet addr:61.xx.yy.zz  Bcast:61.xx.yy.255  Mask:255.255.255.0
          inet6 addr: fe80::3285:a9ff:fea7:a0bb/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:339 errors:0 dropped:0 overruns:0 frame:0
          TX packets:279 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:53807 (52.5 KiB)  TX bytes:23917 (23.3 KiB)
          Interrupt:18 Memory:f7e00000-f7e20000 

eth2      Link encap:Ethernet  HWaddr 00:1b:21:39:1f:e1  
          inet addr:192.168.1.101  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::21b:21ff:fe39:1fe1/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:643 errors:0 dropped:0 overruns:0 frame:0
          TX packets:418 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:72405 (70.7 KiB)  TX bytes:43037 (42.0 KiB)
          Interrupt:16 Memory:f7dc0000-f7de0000 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:12 errors:0 dropped:0 overruns:0 frame:0
          TX packets:12 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:1104 (1.0 KiB)  TX bytes:1104 (1.0 KiB)

My ISP gave me static ip range for internet access: 123.123.123.120-125

(Debian) /etc/network/interfaces:

auto eth1
iface eth1 inet static
address 123.123.123.120
netmask 255.255.255.0
gateway 123.123.123.254
dns-nameservers 8.8.8.8

When I go to an external website to check my ip address, it does not report 123.123.123.120, in fact exernal ip check shows me 23.23.23.45 which is different from what the ISP gave me. I can still access internet normally though.

The Windows machine right beside this Linux machine is setup using 123.123.123.125, and the website exernal ip check shows 123.123.123.125

I suspect that the ISP is using some kind of advanced proxy? Is it possible that only the Linux machine recognizes the real IP but not Windows?

So I did another test, I setup a Windows machine and used same ip 123.123.123.120, I unplugged ethernet cable from Linux machine to Windows machine, run ip check and the ip check reports correctly, it reports 123.123.123.120.

What is causing this weird phenomenon in the Linux machine?

My ISP gave me static ip range for internet access: 123.123.123.120-125

(Debian) /etc/network/interfaces:

auto eth1
iface eth1 inet static
address 123.123.123.120
netmask 255.255.255.0
gateway 123.123.123.254
dns-nameservers 8.8.8.8

When I go to an external website to check my ip address, it does not report 123.123.123.120, in fact exernal ip check shows me 23.23.23.45 which is different from what the ISP gave me. I can still access internet normally though. (Traceroute shows 23.23.23.45 is also owned by my ISP)

The Windows machine right beside this Linux machine is setup using 123.123.123.125, and the website exernal ip check shows 123.123.123.125

I suspect that the ISP is using some kind of advanced proxy? Is it possible that only the Linux machine recognizes the real IP but not Windows?

So I did another test, I setup a Windows machine and used same ip 123.123.123.120, I unplugged ethernet cable from Linux machine to Windows machine, run ip check and the ip check reports correctly, it reports 123.123.123.120.

What is causing this weird phenomenon in the Linux machine?