We had to move a couple of hundred machines from a big Layer 2 broadcast domain into a fully routed network. We also increased the bandwidth available for each server by mounting a new NIC into each server, To avoid synchronisation with service owners, we did not renumber hosts. In addition, there is connectivity to the large Layer 2 domain that is still in place and still has a few thousand hosts in need of migration. In this talk you will learn how we engineered this migration, minimised downtime and ensured the documentation of our network is always up to date. Licensed to the public under http://creativecommons.org/licenses/by/4.0 about this event: https://pretalx.com/denog16/talk/WNS7MA/