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I am confused about the meaning of 192.168.x.x (private network). In DSL bridge mode, my computer has been assigned an IP as part of ISP's private network such as 36.82.x.x. It also can connect to the outside world through an public IP.

In LAN, 192.168.1.1 will be my router, 192.168.1.x is another computers connected to the router (as assigned by DHCP server).

I would like to know what is the rule if I connect to DSL network? I can still see 192.168.1.1 but this is not my own router. Scanning this range shows that there are also several live IPs such as 192.168.1.161, 192.168.1.162, etc. They are not part of my private network.

Edit:

I'm doing nmap scan to 192.168.1.0/24 while in bridging mode. I can see several IPs are alive which are not mine. I don't know much about how bridging to ISP works, but I assume that after bridging, 192.168.x.x are not my private network.

Add Output:

# ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> ...
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
2: eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> ...
    ...
3: wlan0: <NO-CARRIER,BROADCAST,MULTICAST,UP> ...
    ...
5: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> ...
    link/ether f8:d1:11:bc:9f:06 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::fad1:11ff:febc:9f06/64 scope link
       valid_lft forever preferred_lft forever
6: ppp0: <POINTTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> ...
    link/ppp
    inet 125.164.x.x peer 125.164.x.x/32 brd 125.164.x.x scope global ppp0

# traceroute 192.168.1.1
traceroute to 192.168.1.1 (192.168.1.1), 30 hops max, 60 byte packets
 1 125.164.xx.1  
 2 125.164.xx.1
 3 118.98.xx.xx
 4 61.94.xx.xx
 5 61.94.xx.xx
 6 118.98.xx.xx
 7 118.98.xx.xx
 8 192.168.1.1

It requires 8 hops to reach 192.168.1.1.

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4
  • You first two paragraphs make sense but then in your last paragraph it suddenly becomes unclear what you're talking about. Are you still talking about bridging to your ISP? Or are you talking about connecting some other way?
    – David Schwartz
    Dec 26, 2013 at 8:04
  • "my computer has been assigned an IP as part of ISP's private network such as 36.82.x.x." -- That's because you are (mis)using bridge mode on your modem/router, which exposes your PC to the Internet. You need a router in between your ISP and all of your PCs to provide a firewall and NAT for protection. Then this 36.82.x.x IP address would be assigned to the WAN side of the router. 192.168.1.1 is often assigned to the LAN side of the router. The DHCP server in the router should then hand out IP addresses in its assigned range, apparently 192.168.1.xxx.
    – sawdust
    Dec 26, 2013 at 9:35
  • @DavidSchwartz: yes, I'm still talking about bridging to ISP. I use nmap to scan 192.168.1.1-255 and I found several IPs which are not mine.
    – David Bower
    Dec 26, 2013 at 15:55
  • @sawdust: yes, I know that 192.168.1.1 is often assigned to the LAN side. IP in range 192.168.x.x usually are anything belong to me. But when I'm doing bridging to ISP, I can still see 192.168.1.1 IP which is not mine. I would like know who owned or assigned them?
    – David Bower
    Dec 26, 2013 at 15:58

2 Answers 2

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It appears that your ISP uses 192.168.1.0/24 for something inside their network. When you are bridged to their network, you see whatever they've setup machines on their network to see. It's possible that someone messed up and these were intended to be private, but it's also possible that they were intended to be reachable by you.

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  • If his interface to his ISP is a public IP how could he be able to address private IP addresses on the otherside of the dslam? Seems very unlikely
    – 50-3
    Dec 26, 2013 at 19:17
  • 1
    There's nothing magic about private IP addresses inside a single network. This is all taking place inside his ISP's network. It's not unlikely -- it's supposed to work this way. ISPs can, for example, use private IP address space to provide services just to their own customers. (In fact, this is precisely what private IP addresses are for.)
    – David Schwartz
    Dec 26, 2013 at 19:59
  • Would you like to provide some references about how bridging works? I'm interested on the fact that I'm able to access private IP addresses on the otherside of the dslam.
    – David Bower
    Dec 27, 2013 at 6:54
  • Bridging just makes the DSL line invisible and makes your computer an end node in your provider's network. (And it would work without bridging too, so long as they were private IP addresses you didn't use for some other purpose yourself. For example, if you use 192.168.0.0/24, you can probably still reach the provider's 192.168.1.0/24. The only thing special about private addresses is they aren't (supposed to be) reachable across ISPs.)
    – David Schwartz
    Dec 27, 2013 at 7:19
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192.168.x.x is a class C private IP address it allows you to build a network of multiple PCs and uses NAT to route all of them through a single public IP address.

Most ISPs will by default only provide one IP address and unless you plan on only having 1 Device attached to the internet you need to find a way of of connecting multiple devices though 1 IP address.

This is why we have Private IP addresses, they are not owned by anyone and can be used by anyone freely.

The 3 main Private IP ranages are:
10.0.0.0/8
172.16.0.0/12
192.168.0.0/16

I would like to know what is the rule if I connect to DSL network?

There is no one hard and fast rule but best practice is you will be given a single IP address by your ISP (Commonly dynamic) that is assigned to your WAN port on your router. Then your router will build a local private network passed of it's wifi and LAN ports where applicable. This mean on a private network of 192.168.1.0\24 (Subnet mask: 255.255.255.0) you can connect 254 other devices all relying on the one public IP address. This process is very important with IPV4 because we are running out of IP addresses hence IPV6 being rolled out. If every device in the world was assigned a public IP address then we would have run out of IP addresses long ago

Also note if your running a DHCP server for the 192.168.1.0\24 network any device with IP address of 192.168.1.x is in your private network

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3
  • When I'm doing bridging to my ISP network and doing nmap scan to 192.168.1.0/24, I can see several IPs are alive. They are not mine. I would like to know how they works (and maybe how bridging to ISP is done) so I can deduce what are them, who owned them or their real IP if any?
    – David Bower
    Dec 26, 2013 at 16:09
  • Can you add a interface config output for your pc?
    – 50-3
    Dec 26, 2013 at 19:18
  • Yes, of course. I have added interface config output and a traceroute to 192.168.1.1 result in my question.
    – David Bower
    Dec 27, 2013 at 6:49

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