outes' [68]. RFC 4191 defines preferences for both default routers and for more specific routes. Specifically, ICMPv6 Router Advertisement is enhanced with bits that define whether a router is to be consider as a high, medium (default), or low priority default router. With these bits a network administrator can define which routers the hosts should prefer when communicating to the Internet. For the scenarios where more fine-grained router selection is needed, RFC 4191 allows definition of specific routes with the help of the Route Information Option (RIO). This option allows configuration of variable length prefixes and preferences for using the router when sending packets to matching destinations. The preference values for specific routes are the same as for default router configuration [68]. The combination of default and more specific rules allow configurations, for example, where a router is configured to be a low priority default router, but a high priority router for traffic destined to specific networks. This property can be very useful, for example, in cellular networks for implementing a traffic offloading configuration, if a mobile handset is simultaneously connected to a cellular and a local area network. The low priority default router in the cellular network effectively guides the mobile host to prefer local area networks for Internet traffic (provided that the router in the local area network is indicating either medium or high priority default routing services). The router selection influences source address selection procedures, because for egress and ingress filtering reasons use of an IPv6 address configured from one access network on another access network usually does not yield a working connection (see Section 3.5.4). Even if the network would allow asymmetric traffic, the purpose of router prioritization would be partially lost as downlink traffic would likely come via a less preferred router. 3.6 IPv6 Link Types and Models The IPv6 is a layer 3 protocol in the Open System Interconnect (OSI)