How to Get system IP (IPv4) address and convert to string using C# asp.net

Question

On my web application I am using the following function to get System IP

Function

public void SetHostid()
    {
        try
        {
            string ip = "";
            string strHostName = "";
            strHostName = System.Net.Dns.GetHostName();

            IPHostEntry ipEntry = System.Net.Dns.GetHostEntry(strHostName);

            IPAddress[] addr = ipEntry.AddressList;

            ip = addr[1].MapToIPv4().ToString();

            HostId = ip;
            HttpContext.Current.Session["Hostid"] = HostId;
        }
        catch (Exception ex)
        {
            Error_ManagerClass em = new Error_ManagerClass();
            em.WriteError(ex);
        }
    }

It works perfectly because the IP is on the 1 postion of the variable addr (addr[ 1]).

And the problem comes when I try to run the same solution from a different system. function throws an error while trynig to convert IP to string( ip = addr[1].MapToIPv4().ToString(); ) because IP is not in the position number 1.

how can I change the function to work on every computer ??

There is a host file in the c:\windows folder that contains the list of IP. In your case you can use regex to get the IP that matches X.X.X.X. — jdweng
– jdweng, Commented Sep 18, 2017 at 6:53
"to get System IP" ... which one? I understand you want an IPv4 address. But looking at your 2nd example: which one? What should happen on Systems that only support IPv6 ? — Fildor
– Fildor, Commented Sep 18, 2017 at 6:53
if the host has multiple IP adresses, how do you decide which one to take? — Mong Zhu
– Mong Zhu, Commented Sep 18, 2017 at 6:53
Depending on a specific item of an array is surely not the good idea. You need to loop thru the array of IPAddress and check it's property IsIPv4MappedToIPv6 before calling MapToIpv4 method. — Chetan
– Chetan, Commented Sep 18, 2017 at 6:54
The issue is a bit more complicated than "Get my IP-Address". If you want it to "work on every computer" you need to take into account that a computer can have multiple network interfaces (e.g. Ethernet, Wifi, Cell, etc.) and every interface can potentially multiple addresses. A good first start would be to filter potential results with IPAddress.IsLoopback (to get rid of 127.0.0.1 and ::1) and AddressFamily == AddressFamily.InterNetwork to get rid of non-IPv4 Addresses. — Manfred Radlwimmer
– Manfred Radlwimmer, Commented Sep 18, 2017 at 7:08

Roman · Accepted Answer · 2017-09-18 06:56:40Z

2

If you want to get IPv4 only use this code:

var addr = ipEntry.AddressList.Where(ip => ip.AddressFamily == AddressFamily.InterNetwork);

var firstInList = addr.First(); // get first

But you should consider which IP to chose when there are several IP addresses in system.

edited Sep 18, 2017 at 6:56

answered Sep 18, 2017 at 6:54

Roman

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2 Comments

John Wu Over a year ago

Think you need either First or Single to end your LINQ.

Roman Over a year ago

@JohnWu, It gets all IPv4 in system, then you should chose needed one

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