BROCADE IP PRIMER

Chapter 2: TCP/IPHeader Length (HLEN). This is also a 4-bit number. It represents the length ofthe header, but it uses a 32-bit (4-byte) word as its unit. For example, mostheaders are 20 bytes long. To represent this as 4-byte words, this numberwould be 5 (5 x 4 bytes [32-bits] = 20 bytes).ToS with IP Precedence Bits. This is one byte (8 bits). The first three bits arepriority bits. They're used to set IP Precedence. The next four bits describe thepacket's Type of Service (ToS). This allows for special instructions in handlingdifferent types of packets. The last bit is unused.Total Length. This is a 16-bit (2 byte) number. It is the length (in bytes) of theentire packet, including the header and the data that follows.Identifier. This is a 16-bit (2 byte) number that uniquely identifies the packet.Flags. This is a 3-bit field. Should the packet need to be fragmented, it will beindicated here.Frag Offset. This is a 13-bit number. If the packet is too large to fit on a singleframe, it will be fragmented. The offset will help in reassembling the packet.Time-To-Live (TTL). This is an 8-bit (1 byte) value that is set when the packet isgenerated. The idea is that if the packet doesn't get to its destination beforethe TTL expires, the packet is dropped. This is a safeguard to keep the networkfree of orphaned packets (e.g., packets that never get to their destination buthop from router to router to router).Protocol. This is an 8-bit (1 byte) value that indicates the IP Protocol. Thisnumber could represent a Network Layer protocol, or it could represent theTransport Layer protocol that generated the segment (and that will need to processthe segment on the other side). Some well-known values here are: 1(ICMP), 6 (TCP), 17 (UDP), 50 (ESP) and 89 (OSPF).Header Checksum. This is a 16-bit cyclical redundancy check (CRC) of theheader. It allows the receiving side to make sure the header arrived intact.Source IP Address. This is the 32-bit address of the device that created thepacket.Destination IP Address. This is 32-bit address of the device that the packet isto be delivered to.IP Option. This field is typically used for testing and low-level networkdebugging.Data. This is the segment received from the Transport Layer.You can see from this that IP uses 32-bit addresses. These are usually separatedinto four 8-bit numbers separated by periods. They are also usuallyrepresented in decimal (base 10). An example of an IP address might be:192.168.100.1.38 Brocade IP Primer