Public vs. Private IP address?

Discussion in 'Mac OS X Server, Xserve, and Networking' started by Kristina85, Dec 4, 2014.

  1. Kristina85 macrumors member

    Dec 29, 2012
    I have been told that my router needs to allocate private IP addresses to the public one (the one I receive from my university) in order to not disturb the funcionality of the network in the building where I am currently. Would you be able to help me understand this better:

    The appliances which are connected to my router have the following public IP addresses (found by: "what is my IP" on the web):

    The private IP addresses (got them e.g. through Terminal:isconfig) look like the following:

    Now are these really private addresses?

    My internet works fine now but I need to make sure the setting is right (otherwise I will get trouble from IT people)
    I have also found the following information: ranges and the amount of usable IP's are as follows: -
    Addresses: 16,777,216 -
    Addresses: 1,048,576 -
    Addresses: 65,536

    If this is true, then my private IP addresses are not really private? And if so, what are the problems that I could cause exactly in this regard?
    Can I set up private address in a router manually? How?

    Thanks for helping me make sense of this!
  2. talmy macrumors 601


    Oct 26, 2009
    If your subnet mask is you are OK, if it is or less then no.

    You should have a router between your schools network and your private network. The router does address translation which makes all your private devices appear as a single device on the schools network.

    What is strange here is you mention two public addresses rather than just one which seems like you have two routers. Also most routers will, by default, assign private addresses in the 192.168.1.* range. 144.8.127.* is not a range of private addresses and will cause problems if you need to access a public site in that range. So it looks like your router has a very non-standard (and very wrong) configuration. Whatever it is, try a factory reset. That should restore it to issuing addresses in a private range.
  3. brentmore macrumors 6502


    Jul 19, 2002

    Your university gave you 1 public IP address. Your job is to use your own router to divvy up your network so that all of your devices can 'share' that 1 public IP address and get their own private address. You router will most likely do this using Network Address Translation (NAT). It will assign a private IP like 192.168.1.x to each of your devices. As far as the rest of the world is concerned, you only have one address (the public one), but YOU know that you have lots of other (private) addresses on your network.

    So if you do "what's my ip" from any of the devices on your personal network, you should get the same answer from all of them. If that's not the case, it's incorrectly configured and IT will probably let you know about it.
  4. monokakata, Dec 5, 2014
    Last edited: Dec 5, 2014

    monokakata macrumors 68000


    May 8, 2008
    Hilo, Hawai'i
    So -- considering your other thread -- you're making progress with the university IT folks?

    That's good.

    You're using a Mac, right? Why don't you go to System Preferences, then to Network, and then report what IP address is, and then what Router address is.

    For example, the Mac I'm typing this on is at and my router (gateway to the outside) is This is a standard setup.

    Someplace in your router's setup procedures (what kind of router is it?) there will be a place for you to set the range of internal addresses. As Talmy says, a 144.x.x.x range is strange. Almost always the only choices you get are 192.x.x.x. and 10.x.x.x.
  5. sevoneone, Dec 6, 2014
    Last edited: Dec 6, 2014

    sevoneone macrumors 6502

    May 16, 2010
    The school likely has multiple ISP connections so they can load balance and more easily allocate bandwidth around the network as needed, so seeing a different public IP at different times and on different devices on what's my ip is not that strange. It just means one device could be using a slightly different egress route out to the internet.

    What is odd is the 144.8.127.x addresses on each device. That either means your router's ip configuration is way off, or it isn't acting as a router at all and is basically just being a switch/bridge.

    Though if the school is assigning public IPs to every device, that would most likely mean that there is no NAT happening, each device is just directly on the internet, which seems a little crazy from a security standpoint, but maybe not if it is a residential situation and the IT dept. wants all responsibility for security and privacy on the user.
  6. Kristina85 thread starter macrumors member

    Dec 29, 2012
    Talmy, Sevoneone, Monokakata and Brentmore,

    I am thankful to your responses - all of you really helpful. Let me clarify a few things and respond to some of your posts, hopefully you'll be able to explain this better to me:

    The situation with my router (I think it must be the reason for the confusing situation). I am currently using a device which is very weirdly configured. It is a device which you will receive here in Japan from one of the large internet when you use their cable.
    See here for the scheme:;456;402

    If I plug the Ethernet cable into one of the LAN ports (LAN1 to LAN4), I am able to receive a wifi signal for all my devices and use them (there's an internal cable that connects VSDL with LAN ports-setting by the company). The trouble is that the IP addresses look at the follows:

    Mac PC:
    public address: (www my IP)
    IPv4 address:
    Subnet Mask

    public address: (www my IP)
    IP Address:
    Subnet Mask
    DNS: 144.1.5,

    Now if insert the Ethernet cable into the VSDL plug (i.e. the way it normally shoud be), I get the following:
    Mac PC:
    Public: none
    IPv4 address:
    Subnet Mask

    Here you see that finally the router is assigning the proper Private Address to my laptop. The trouble is that now, even though connected to the router, I cannot download anything.
    I get the following messages in my broswers:

    1. server not found (after trying to connect for a long time)
    2. unable to find the proxy server (this is after I try to connect with the proper proxy setting...the only way how to connect to the network and start using the web is through a 8080 port, a proxy server and a html log-on website).

    In sum, with a weird setting (Ethernet into lan1), internet works, can connect my devices but the setting is not right and if I understand it correctly, this could cause trouble to the network.
    With the Ethernet cable inserted properly I cannot use the internet.

    I was explained that the setting has to be right (one Public address...and then the router assigns many private ones to the necessary devices), but I am unable to set it up this way. One of the people who lived in this apartment before told me that he managed to do it only after buying a modem and flashing it with the following software:

    I would be most grateful to hear your advice.


    Brentmore, that's absolutely correct. Yes, I am trying to get it set up the way you have suggested, but not able to do so. please see below and let me know what you think.
  7. talmy macrumors 601


    Oct 26, 2009
    I see now -- you don't have a router but a combination modem/router. The modem part is why it doesn't work when connected to the VSDL port. If you just use the LAN ports, it's nothing more than a Ethernet switch and won't do the address translation. Pure routers seem to be hard to find -- you may need to get a "wireless router" which is a combination of router and wireless access point, and then turn the wireless access point off.
  8. Kristina85 thread starter macrumors member

    Dec 29, 2012
    once again, thank you for your response.
    I am glad to report that I have managed to set up everything properly now.
    There are additional questions though I hoped to get help from you. devices are properly connected now (having private addresses in the proper range with one public address). However, Ipad + Iphone + Iphone share one private address while my two Macs share another.
    Is this normal?
    It looks something like this: (for Ipad, iphone) for my two PCs.
    I thought that it should be one public address for all devices?

    2. I managed to set up my network on a router (a Japanese setting) I bought second hand. There were no instructions as to what the SSID should be, neither the password.
    I connected to the router through an ethernet cable..and the following: in a browser. Looking at the maker's instructions, it was enough to enter "root" and no password ..and I could get into the router settings. There I found both SSID for the wireless connection + a range of four keys...and one of them, the first, worked. It is a WEP64 key..but I am confused as to the security of the whole thing. Is this normal to be set up in this way? Can someone from the outside connect to my router? (i.e. through wifi..and see the settings in this way as well?)

    3. This is a university network..and they allow us to use it by loging a proxy..and 8080 port (I've heard that this is rare..but this is the proxy's port I need to enter in order for all my devices to work).

    4. Last but not least, I am confused as to how things get updated in Itunes (maybe you will not know), especially when it comes to podcasts. I can update on this network my podcasts on my Mac, but trying to do the same on an Ipad (using Podcast application) I am unable to do so. It's constantly refreshing..though it is capable to download the latests titles of the podcasts (i.e. not data, but at least the information that there is a new one). Can this be somehow related to the fact that I am connecting to the network through 8080 port?

    One again, thank you for your help.
  9. talmy macrumors 601


    Oct 26, 2009
    If the router is at then all your devices should be 192.168.11.* and different for each one. There should only be one public address for everything, basically the address of your router's WAN port.

    So I'm still baffled by the setup.

    It's been probably 20 years since I last used a proxy. I don't remember anything about it. And I do know I wasn't using it behind a router. All Internet requests were sent through it. Basically a crude alternative to NAT and a router.

    It sounds like you may have a proxy for HTTP service. I don't know the protocol used for podcast transmission, but it might be on another (and effectively blocked) port.

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