Our internal and external domain are the same, for this discussion let's assume that domain is "somedomain.com". We switched webhost and we started to have issues with not being able to reach our company website "www.somedomain.com" from the internal network. If your internal and external domains are the same, then your internal computers will look for "www.somedomain.com" to be inside your network, even if your DNS forwarders are working fine. If you are having same issue then you have to know that there are couple of things you have to do in your internal DNS so that your internal computers can resolve your website, www.somedomain.com.
In your Internal Network on your DNS server create a new Forward Lookup Zone. Go through the new Zone creation wizard. When asked for name, name this Zone www
After the zone is created, created a Host (A) Record in this forward lookup zone that points to the IP address of the host that is hosting your website. You can leave the name blank, your host record would look something like this.
same as parent folder Host (A) 184.108.40.206 (this is the IP address of the external host)
Please note, that if you know the FQDN (fully qualified domain name) of the host that is hosting your external website then you don't have to create the new zone, you can just create an Alias (Cname) record in the Forward lookup zone of your domain pointing to the FQDN of that host.
www CNAME virtual.sitehost.com