Undefined reference to...

Having this class

....
// class for managing the low level packets
class Packet
{
private:
    char type;
    virtual void writeBuffer();

protected:
    char buffer[MAX_SIZE];
    size_t real_size;

public:
    // when we construct it directly we hmust give in the stream
    Packet(char _type) : type(_type) {}
    friend ostream& operator<<(ostream& s, const Packet& c);
    // virtual bool operator==(const Packet&) const = 0;
    static Packet parseStream(const Stream&);
    // implement a sending mechanism inside here
    void sendMulticast();
};

What I just want to do is that this is implemented by the class
inheriting from Packet.

    virtual void writeBuffer();

But actually every time I try to virtualize something I get

Undefined symbols:
  "typeinfo for Packet", referenced from:
      typeinfo for Beaconin Beacon.o
  "vtable for Packet", referenced from:
      Packet::Packet(char)in Beacon.o
      Packet::Packet(Packet const&)in Packet.o

Now I found this link
http://lifecs.likai.org/2009/03/c-undefined-vtable-symbol.html

which was the only clear explanation, but still I don't get it, the ONLY
class that inherits from Packet HAS writeToBuffer implemented.
So what can that be?

Without any virtuals everything works perfectly..