Large Flat Network with /16 Mask

What steps would you take to move from a L2 flat 10.x network to a routable L3 network.  The gear is 15 yrs old and having the obvious issues that come with a flat network.
John LeBrunExecutive Director, Client Solutions and ServicesAsked:
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Don JohnstonInstructorCommented:
That's kind of a wide ranging question with not much to go on.

Like how large is the network?  i.e. how many switches and/or hubs?  How many hosts?

Can you do a migration or does this have to be a hard cut?

Are you using DHCP for address assignment?
David Johnson, CD, MVPOwnerCommented:
do you in fact have the available IP Addresses that are not in the private IP spaces? Getting a large block of ipv4 addresses these days tends to be expensive.
atlas_shudderedSr. Network EngineerCommented:
The most challenging thing that you are going to face is getting the original cut to layer 3.  When making that cut, you are going to have to plan for the segregation and loss of connectivity that segments normally use.  If you are using a 10. with a /16 you will have plenty of optional space in the remainder of the overhead /8.  My suggestion would be to leverage that availability and utilize /24's or /23's for your new subnets.  That being said, the first thing that you will want to do is establish your layer three gateway.

If you have a router already at the edge, then you are partway there already.  You will want to set up your first VLAN on the head end switch and then establish a trunk to the router, using sub interfaces on the router.  Extend the VLAN out to the rest of your network switches and then set up an SVI on a far end switch to test gateway reachability and routing.  Assuming you are using DHCP, create your new subnet scope on your server.  Once this is completed, configure your DHCP helper on your network gateway at the router or on an SVI on your headend switch.  Be sure to test DHCP and routing using a laptop before any production migrations.  From there, you can start moving your first block of hosts over to their new VLAN (I would suggest not moving servers unless absolutely unavoidable or necessary).  Rinse, Repeat.

In the event you are planning on extending layer three to your switching infrastructure, things will be a bit more complicated.  If this is your desired route, post back and I can give you some thoughts on that.

All this being said, some caveats.

1.  Make sure you test each step as you go.  It's more work but it's better that you catch the problems yourself rather than have orphaned hosts on the network.
2.  Try to keep things to workstations only as much as possible.  If branching out to other equipment, keep like and common use together as much as possible.
3.  I can't stress this one enough, do everything you can to leave servers and storage where it is, especially DC's and NAS devices.  They don't like to be moved around, even if it is just a logical change and the headaches and their impact tend to grow exponentially when you start moving them around.

Hope that helps

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John LeBrunExecutive Director, Client Solutions and ServicesAuthor Commented:
Thanks everyone,  I'm new to this and love the feedback. I know it was an ambiguous question,  but there is only 1 public IP being NATed by a FW. All other IP's are 10.x space /16 mask. OVERKILL.  There is no easy way other than redesigning the entire network.
John LeBrunExecutive Director, Client Solutions and ServicesAuthor Commented:
Thanks everyone!  Great feedback and pretty obvious. Looking at the MDF and IDF looks daunting however, it needs to be done....whenever.  Thanks everyone, great feedback as I'm new to this service.
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