Jenkins uses a TCP port to communicate with agents launched via the JNLP protocol, such as Windows-based agents. As of Jenkins 2.0, by default this port is disabled. You need to verify that JNLP is enabled and TCP port is defined in the Global Security section.
Go to Manage Jenkins -> Configure Global Security, and then scroll down to Agents, define a static TCP port for JNLP agents (random ports making it difficult to manage firewall rules on agents hosts ’cause port is chosen during the boot of Jenkins master), click Save and get back to agent’s configuration (try to connect the agent to Jenkins by clicking the Launch button, and then verify that agent’s status is “Connected via JNLP agent”)
UseIT | Roman Levchenko 
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