Whereas cone NATs are used in small and domestic private networks, in most of the corporative LANs the NATs are symmetric. When a peer is behind a symmetric NAT, only can receive blocks from a public node if has sent at least a block to it. Another drawback of using symmetric NATs is that each diﬀerent “connection”∗ ∗Using UDP the concept of connection does not make sense. However, we can use this name to reefer to the tuple ((public IP address, public port), (private IP address, private port)), where the public endpoint points to a public peer and the private endpoint points to the private peer. has a diﬀerent public endpoint. Besides this problem, some networks administrator refuses UDP traﬃc because it can congest their networks easier than using TCP traﬃc.
One way of solving this problem is creating a diﬀerent team for the private network, in a similar way as was explained in Section 7.5, but using a peer located in the public network (see Figure 5). Notice that in this conﬁguration, only a TCP connection need to be established through the NAT.