principle of global or end-to-end reachability, which means that any Internet user can transparently exchange traffic with any other Internet user. Therefore, a network is connected to the Internet if and only if it buys transit, or peers with every other network which also does not purchase transit (which together constitute a "default free zone" or "DFZ"). Motivations for peering Peering involves two networks coming together to exchange traffic with each other freely, and for mutual benefit. This 'mutual benefit' is most often the motivation behind peering, which is often described solely by "reduced costs for transit services". Other less tangible motivations can include: Increased redundancy (by reducing dependence on one or more transit providers). Increased capacity for extremely large amounts of traffic (distributing traffic across many networks). Increased routing control over one's traffic. Improved performance (attempting to bypass potential bottlenecks with a "direct" path). Improved perception of one's network (being able to claim a "higher tier"). Ease of requesting for emergency aid (from friendly peers). Physical interconnections for peering The physical interconnections used for peering are categorized into two types: Public peering – Interconnection utilizing a multi-party shared switch fabric such as an Ethernet switch. Private peering – Interconnection utilizing a point-to-point link between two parties. Public peering Public peering is accomplished across a Layer 2 access technology, generally called a shared fabric. At these locations, multiple carriers interconnect with one or more other carriers across a single physical port. Historically, public peering locations were known as network access points (NAPs). Today they are most often called exchange points or Internet exchanges ("IXP"). Many of the largest exchange points in the world can have hundreds of participants, and some span multiple buildings and colocation facilities across a city. Since public peering allows networks interested in peering to interconnect with many other networks through a single port, it is often considered to offer "less capacity" than private peering, but to a larger number of networks. Many smaller networks, or networks which are just beginning to peer, find that public peering exchange points provide an excellent way to meet and interconnect with other networks which may be open to peering with them. Some larger networks utilize public peering as a way to aggregate a large number of "smaller peers", or as a location for conducting low-cost "trial peering" without the expense of provisioning private peering on a temporary basis, while other