diff --git a/server.go b/server.go
index b93be96..f67d7ef 100644
--- a/server.go
+++ b/server.go
@@ -9,9 +9,78 @@ import (
 	"gopkg.in/irc.v3"
 )
 
+type ircError struct {
+	Message *irc.Message
+}
+
+func newUnknownCommandError(cmd string) ircError {
+	return ircError{&irc.Message{
+		Command: irc.ERR_UNKNOWNCOMMAND,
+		Params: []string{
+			"*",
+			cmd,
+			"Unknown command",
+		},
+	}}
+}
+
+func newNeedMoreParamsError(cmd string) ircError {
+	return ircError{&irc.Message{
+		Command: irc.ERR_NEEDMOREPARAMS,
+		Params: []string{
+			"*",
+			cmd,
+			"Not enough parameters",
+		},
+	}}
+}
+
+func (err ircError) Error() string {
+	return err.Message.String()
+}
+
 type conn struct {
-	net net.Conn
-	irc *irc.Conn
+	net        net.Conn
+	irc        *irc.Conn
+	registered bool
+	nick       string
+	username   string
+	realname   string
+}
+
+func (c *conn) handleMessageUnregistered(msg *irc.Message) error {
+	switch msg.Command {
+	case "NICK":
+		if len(msg.Params) != 1 {
+			return newNeedMoreParamsError(msg.Command)
+		}
+		c.nick = msg.Params[0]
+	case "USER":
+		if len(msg.Params) != 4 {
+			return newNeedMoreParamsError(msg.Command)
+		}
+		c.username = "~" + msg.Params[0]
+		c.realname = msg.Params[3]
+		c.registered = true
+	default:
+		return newUnknownCommandError(msg.Command)
+	}
+	return nil
+}
+
+func (c *conn) handleMessage(msg *irc.Message) error {
+	switch msg.Command {
+	case "NICK", "USER":
+		return ircError{&irc.Message{
+			Command: irc.ERR_ALREADYREGISTERED,
+			Params: []string{
+				c.nick,
+				"You may not reregister",
+			},
+		}}
+	default:
+		return newUnknownCommandError(msg.Command)
+	}
 }
 
 type Server struct{}
@@ -19,26 +88,27 @@ type Server struct{}
 func (s *Server) handleConn(netConn net.Conn) error {
 	defer netConn.Close()
 
-	conn := conn{netConn, irc.NewConn(netConn)}
+	c := conn{net: netConn, irc: irc.NewConn(netConn)}
 	for {
-		msg, err := conn.irc.ReadMessage()
+		msg, err := c.irc.ReadMessage()
 		if err == io.EOF {
 			break
 		} else if err != nil {
-			return err
+			return fmt.Errorf("failed to read IRC command: %v", err)
 		}
 		log.Println(msg)
 
-		switch msg.Command {
-		default:
-			err = conn.irc.WriteMessage(&irc.Message{
-				Command: irc.ERR_UNKNOWNCOMMAND,
-				Params: []string{
-					"*",
-					msg.Command,
-					"Unknown command",
-				},
-			})
+		if c.registered {
+			err = c.handleMessage(msg)
+		} else {
+			err = c.handleMessageUnregistered(msg)
+		}
+		if ircErr, ok := err.(ircError); ok {
+			if err := c.irc.WriteMessage(ircErr.Message); err != nil {
+				return fmt.Errorf("failed to write IRC reply: %v", err)
+			}
+		} else if err != nil {
+			return fmt.Errorf("failed to handle IRC command %q: %v", msg.Command, err)
 		}
 	}