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This looks like a question of setting up your IDE. Have you tried looking on a forum for you IDE to find out how to use packages between projects?

Another solution is to put the client.Person.class file in the Server.jar file.

What folder contains the client folder?
Put the path to that folder in the classpath:
java -cp .;<PATH-TO-FOLDER-WITH-client-FOLDER> ...

The javac and the java programs use the classpath to find...

import client.Person;
Does your classpath point to the folder containing the client folder?

client.Person p = (client.Person)ois.readObject();

Is the Person.class file on the classpath for the Server? Is the Person class in a package? Your source does not show it.
Can you post the full text of the error message showing the source line...