PropertyFileSnitch 

Determines the location of nodes by rack and datacenter.

This snitch determines proximity as determined by rack and datacenter. It uses the network details located in the cassandra-topology.properties file. When using this snitch, you can define your datacenter names to be whatever you want. Make sure that the datacenter names correlate to the name of your datacenters in the keyspace definition. Every node in the cluster should be described in the cassandra-topology.properties file, and this file should be exactly the same on every node in the cluster.

The location of the cassandra-topology.properties file depends on the type of installation:
Package installations /etc/cassandra/cassandra-topology.properties
Tarball installations install_location/conf/cassandra-topology.properties

Procedure

If you had non-uniform IPs and two physical datacenters with two racks in each, and a third logical datacenter for replicating analytics data, the cassandra-topology.properties file might look like this:
Note: datacenter and rack names are case-sensitive.
# datacenter One

175.56.12.105=DC1:RAC1
175.50.13.200=DC1:RAC1
175.54.35.197=DC1:RAC1

120.53.24.101=DC1:RAC2
120.55.16.200=DC1:RAC2
120.57.102.103=DC1:RAC2

# datacenter Two

110.56.12.120=DC2:RAC1
110.50.13.201=DC2:RAC1
110.54.35.184=DC2:RAC1

50.33.23.120=DC2:RAC2
50.45.14.220=DC2:RAC2
50.17.10.203=DC2:RAC2

# Analytics Replication Group

172.106.12.120=DC3:RAC1
172.106.12.121=DC3:RAC1
172.106.12.122=DC3:RAC1

# default for unknown nodes default =DC3:RAC1