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------------------------------------------------------------------------------No. of Routes: 1 =============================================================================== R1#
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R4# show router route-table =============================================================================== Route Table (Router: Base) =============================================================================== Dest Prefix Type Proto Age Pref Next Hop[Interface Name] Metric ------------------------------------------------------------------------------ Local Local 01d10h28m 0 Ethernet 0 ------------------------------------------------------------------------------No. of Routes: 1 =============================================================================== R4#
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The routers use these entries to keep forwarding packets based on the L3 addressing until the packet gets to a router that is local to the destination, meaning that it is on the same local network segment as the end station (e.g., the same Ethernet segment). once the packet reaches the local router, the packet is then forwarded to the endstation using Ethernet framing and forwarding methods discussed in earlier chapters. In the next section, we discuss the structure of an IP packet, its unique fields, and how they are used to assist with the addressing and forwarding functions.
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5.2 The IP Header
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Just as with Ethernet frames, there is a header for IP packets that contains very specific and important fields. Unlike the simplicity of the Ethernet header, however, there are numerous fields in an IP header. These fields provide important functionality to the IP layer that is not available to Ethernet. The IP header is shown in Figure 5.2, and the description of these fields follows.
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5.2 T H E I P H E A D E r
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Figure 5.2 An IP packet header. This header is for version 4 of IP; IP version 6 has a different header, but it is not yet in widespread use.
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0 Version 4 IHL 8 Type of Service Flags Protocol Source IP Address Destination IP Address Options Padding 16 19 Total Length Fragment Offset Header Checksum 31
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Version Always set this to the value 4, which is the current version of IP. (note that there is a current version 6, but it is not yet in wide deployment. Version 6 IP has a different header.) IHL (IP Header Length) A 4-bit field that contains the number of binary words (a word is 32 bits or 4 bytes) forming the header, usually five. This field is required because an IP packet can have options that increase the size of the header from 20 octets (bytes) up to a maximum of 60 octets (bytes). 60 bytes is the maximum because with a 4-bit field, the maximum value is 23 + 22 + 21 + 20 = 8 + 4 + 2 + 1 = 15, and 15 4 bytes (word) = 60 bytes. The record route option can take up all 60 bytes when the recording router hops in a large network. ToS (Type of Service) Also now known as DSCP (Differential services Code Point). The Tos is an 8-bit field, usually set to 0, but that may indicate particular Qos needs from the network. The DsCP defines the way routers should queue packets while they are waiting to be forwarded and in some cases provides for packets to be forwarded along different paths based on priority. Total Length A 16-bit field specifying the total length of the packet, including the header, in octets (bytes). The combined length of the header and the data can be at most 65,535 octets because this is the largest possible decimal number that can be described by 16 bits in binary. Identification A 16-bit number that, together with the source address, uniquely identifies this packet; used during the reassembly of fragmented datagrams. IP allows an intermediate router to break apart an IP packet into smaller pieces in
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case a large packet needs to be forwarded over a network that supports only smaller packets. For example, a packet originating from an Ethernet network that might be 1,500 bytes may need to get forwarded over a PPP network that only supports frames 500 bytes in size. The ID field allows for packets to be broken up and then reassembled at the destination. Flags sequence of two flags (one of the 3 bits is unused) that controls whether routers are allowed to fragment a packet (i.e., the Don t Fragment, or DF, flag), and to indicate the parts of a packet to the receiver via the more Fragment (mF) flag. Fragmentation Offset offset from the start of the original sent packet, set by any router that performs IP fragmentation. Unused if fragmentation is not performed. Time to Live (TTL) number of hops/links that the packet may be routed over, decremented by most routers (used to prevent accidental routing loops). recall our earlier discussion of Ethernet broadcast storms and how Ethernet frames can circulate forever because there is no mechanism to indicate to Ethernet switches that a frame should be discarded. The TTL field ensures that all IP packets have a limited lifetime. Protocol An 8-bit field that indicates the type of transport data being carried (e.g., 6 = TCP, 17 = UDP). Header Checksum This is a field for detecting errors in the IP header. The checksum is calculated by the transmitting router based on the contents of the IP header. It is calculated again by the receiver and compared with the value in the header. If they are different, the packet is discarded. The checksum is updated whenever the packet header is modified by a router (e.g., to decrement the TTL). Source Address 32-bit IP address of the original sender of the packet. Destination Address 32-bit IP address of the final destination of the packet. Options not normally used, but when used, the IP header length is greater than five 32-bit words to indicate the size of the options field. There are many options, but in modern networks most of them are no longer used. originally one of the most common was the strict source route or loose source route. These options allowed for the sending station to specify to intermediate stations how the packet was to be routed. modern routing protocols alleviate most of the need for these and other options. The padding field is used to make sure that the header ends on a 32-bit boundary in the event that options are used. You should make sure that you become familiar with the fields of the IP header especially the TTL, protocol, and addressing fields and understand why
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