Apex Class:

public class IPAddressController
{
    public string IPAddress {get;set;}
    
    public IPAddressController() {
        
        IPAddress = ApexPages.currentPage().getHeaders().get('True-Client-IP');
        //Get IP address when no caching (sandbox, dev, secure urls)
        if (String.isBlank(IPAddress)) {
            IPAddress = ApexPages.currentPage().getHeaders().get('X-Salesforce-SIP');
        }  
        if (String.isBlank(IPAddress)) {
            IPAddress = ApexPages.currentPage().getHeaders().get('X-Forwarded-For');
        } 
    }
}

Visualforce Page:

<apex:page controller="IPAddressController">
    Your Login IP Address : {!IPAddress}
</apex:page>

Output: