Ethernet_II, 802.3, 802.2 vs. SNAP
There are four types of ethernet frame type:
Ether_II,
802.3,
802.2,
SNAP
Ether_II: has a field named "protocal", which specifys the upper layer(network layer)protocal(e.g. IP, IPX, etc.)
802.3: without a "protocal" field but has a "length"
802.2: Since the 802.3 Ethernet frame cannot by itself identify the upper-layer (Network)protocol, it obviously needs some help. The IEEE defined the 802.2LLC specifications to provide this function and more. an 802.2 frame is an 802.3 frame with the LLC information in the data field of the header. 802.2 frame is identified by "Dest. SAP" and "Source SAP" fields, these two fields specifys the upper layer protocal
SNAP: The SNAP frame has its own protocol field to identify the upper-layer protocol.This is really a way to allow an Ethernet_II Ether-Type field to be usedin an 802.3 frame. SNAP frame is identified by its "Dest. SAP" and "Source SAP" fields, always have the value of "0xAA" - SNAP, the upper layer protocal has already been specified in "protocal" field
In short words:
originally we have Ether_II or 802.3(created by Novell, with no Type field, then accepted by IEEE), then IEEE introduced 802.2 and SNAP to modify 802.3 so 802.3 can identify upper level protocol. when we say 802.2 frame, we mean a 802.3 frame with 802.2 header; when we say SNAP frame, we mean on top of 802.3, we have SNAP header. In the meantime, SNAP is a
special 802.2 frame with certain values on SSAP, DSAP, and Command fields.
Oct 27, 2004
Oct 8, 2004
what IS this prisoner.iana.org?
Well, once RFC 1918 (and its predecessors, actually) came out, the IANA -- the old name, recall, for the folks in charge of handing out IP address blocks -- realized that they needed a "placeholder" in-addr.arpa zone for the three ranges of non-routable addresses. So they put zones named 10.in-addr.arpa, 16.172.in-addr.arpa, and 168.192.in-addr.arpa on a three DNS servers named blackhole-1.iana.org, blackhole-2.iana.org and prisoner.iana.org, at IP addresses 192.175.48.6, 192.175.48.42, and 192.175.48.1, and prisoner is set as the primary DNS server for the zones. Thus, if one of your systems with a 192.168.x.x address tries to register its PTR record then it will, unless you have a local DNS server with a 168.192.in-addr.arpa zone, end up trying to register with prisoner.iana.org -- which will reject the request. The bottom line is, don't worry about it in most cases. In one case, however, you MIGHT worry about it, if you were running an intranet with a dialup connection to the Internet.
copy from Expert-Exchange
copy from Expert-Exchange