Switching from an old network service provider to a new network service provider

Have a question, my company is switching over from an old network service provider Verizon to ATT and has been assigned to prepare a checklist what needs to be monitored and/or done. The switch over will be done per location. I have the following ideas as enumerated below but would like to find out if you have any suggestions, inputs and recommendations to handle this switch over safely and in a most efficient manner. I'm a systems admin person and routing and networking are not my strongest suit, so your input would be much appreciated.

If there are any components that I might have missed, please let me know also.  

My Idea: I'm thinking that the external network should be the (only) major concern and internally everything should be unaffected. We have to ensure that the external DNS record will be updated to the new service provider's IP address (which is handled by the vendor). That being said, internally, I think the IP address update should only be done in the email, vpn settings, firewall and external sites areas (ie, such as www.company.com dns record and ftp sites' dns records to reflect the new service provider's IP address. Is there something that I'm missing? And, are there internal components (ie, Active Directory, DNS..etc) that I need to worry about? Would like to find out if this turn over will also impact Active Directory or the internal components in our environment.

Appreciate all of your input on this.
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ToxaconConnect With a Mentor Commented:
Well, it greatly depends on your internal network using private addressing. If it does, you have to update only external references, firewall outer leg address and modifying the routing table, incoming email address (if hosting it internally) and possibly dns forwarders in AD DNS.
Agree with Toxacon, assuming you are using private IP addressing for your network, then the only things to worry about are external-facing devices (firewall) and DNS.

If you have any control over the timing of the cut-overs, I would STRONGLY recommend you schedule them for a Friday evening.

Why, you ask?
1. DNS changes can take up to 48 hours to propagate.
2. If something does go wrong, you've got all weekend to fix it.

This way you avoid having downtime during business hours.

I've done alot of network conversions, even the simplest ones can go wrong.
mmoyaAuthor Commented:
For curiosity sake and if our network does not use private addressing, then what steps are needed then? Thank yoU!
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bcrosby007Connect With a Mentor Commented:
You really have to use private networking (10.x.x.x, 192.168.x.x, 172.16.x.x - 172.16.32.x) for  your internal network unless you only have a few client machines. So, your internal network probably wont change.
To give you a good list, you must provide more information. Do you host your own email server. Own web server. Own firewall/ router. Basically, anything that connects to the outside world statically.
ToxaconConnect With a Mentor Commented:
If your network does not use private addresses, you very likely have to change every single IP address in your internal network, including DHCP scopes, Active Directory subnets etc.

Usually the Service Provider RIPE owns rights to the public block and only assigns IP block to its own customers so changing the SP means changing internal IPs as well, if not private.
mmoyaAuthor Commented:
Thank you for the input. Just confirmed and we are using private IP addresssing..

To bcrosby007, yes, we host your own email server, own web servers and own firewall/ router.
bcrosby007Connect With a Mentor Commented:
Ok. Then you are going to have to update your DNS registrar with the appropriate IP information for your MX recort for mail, plus your A records for all of your other public ip's. Then you will have to change your firewall mapping from the new public ips to your internal private ip's. All in all, it is a pretty seamless operation. @KDearing was correct that it could take the internet dns registrars up to 48 hours to update to your new IP scheme, but i have never seen it take that long. So, some of your clients may get email delivery delays on their end.
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