At this point we have concluded the basic mechanics of IPv4. Still to come is a discussion of how IP routers build their forwarding tables. This turns out to be a complex topic, divided into routing within single organizations and ISPs – 9   Routing-Update Algorithms – and routing between organizations – 10   Large-Scale IP Routing.

But before that, in the next chapter, we compare IPv4 with IPv6, now twenty years old but still seeing limited adoption. The biggest issue fixed by IPv6 is IPv4’s lack of address space, but there are also several other less dramatic improvements.


Exercises are given fractional (floating point) numbers, to allow for interpolation of new exercises. Exercise 6.5 is distinct, for example, from exercises 6.0 and 7.0. Exercises marked with a ♢ have solutions or hints at 24.7   Solutions for IPv4.

1.0. Suppose an Ethernet packet represents a TCP acknowledgment; that is, the packet contains an IPv4 header and a 20-byte TCP header but nothing else. Is the IPv4 packet here smaller than the Ethernet minimum-packet size, and, if so, by how much?

2.0. How can a receiving host tell if an arriving IPv4 packet is unfragmented? Hint: such a packet will be both the “first fragment” and the “last fragment”; how are these two states marked in the IPv4 header?

3.0. How long will it take the IDENT field of the IPv4 header to wrap around, if the sender host A sends a stream of packets to host B as fast as possible? Assume the packet size is 1500 bytes and the bandwidth is 600 Mbps.

4.0. The following diagram has routers A, B, C, D and E; E is the “border router” connecting the site to the Internet. All router-to-router connections are via Ethernet-LAN /24 subnets with addresses of the form 200.0.x. Give forwarding tables for each of A♢, B, C and D. Each table should include each of the listed subnets and also a default entry that routes traffic toward router E. Directly connected subnets may be listed with a next_hop of “direct”.


5.0. (This exercise is an attempt at modeling Internet-2 routing.) Suppose sites S1 … Sn each have a single connection to the standard Internet, and each site Si has a single IPv4 address block Ai. Each site’s connection to the Internet is through a single router Ri; each Ri’s default route points towards the standard Internet. The sites also maintain a separate, higher-speed network among themselves; each site has a single link to this separate network, also through Ri. Describe what the forwarding tables on each Ri will have to look like so that traffic from one Si to another will always use the separate higher-speed network.

6.0. For each IPv4 network prefix given (with length), identify which of the subsequent IPv4 addresses are part of the same subnet.


6.5. Convert the following subnet masks to /k notation, and vice-versa:

(d).♢ /20
(e). /22
(f). /27

7.0. Suppose that the subnet bits below for the following five subnets A-E all come from the beginning of the fourth byte of the IPv4 address; that is, these are subnets of a /24 block.

  • A: 00

  • B: 01

  • C: 110

  • D: 111

  • E: 1010

(a). What are the sizes of each subnet, and the corresponding decimal ranges? Count the addresses with host bits all 0’s or with host bits all 1’s as part of the subnet.
(b). How many IPv4 addresses in the class-C block do not belong to any of the subnets A, B, C, D and E?

8.0. In 7.9   Address Resolution Protocol: ARP it was stated that, in newer implementations, “repeat ARP queries about a timed out entry are first sent unicast”, in order to reduce broadcast traffic. Suppose multiple unicast repeat-ARP queries for host A’s IP address fail, but a followup broadcast query for A’s address succeeds. What probably changed at host A?

9.0. Suppose A broadcasts an ARP query “who-has B?”, receives B’s response, and proceeds to send B a regular IPv4 packet. If B now wishes to reply, why is it likely that A will already be present in B’s ARP cache? Identify a circumstance under which this can fail.

10.0. Suppose A broadcasts an ARP request “who-has B”, but inadvertently lists the physical address of another machine C instead of its own (that is, A’s ARP query has IPsrc = A, but LANsrc = C). What will happen? Will A receive a reply? Will any other hosts on the LAN be able to send to A? What entries will be made in the ARP caches on A, B and C?

11.0. Suppose host A connects to the Internet via Wi-Fi. The default router is RW. Host A now begins exchanging packets with a remote host B: A sends to B, B replies, etc. The exact form of the connection does not matter, except that TCP may not work.

(a). You now plug in A’s Ethernet cable. The Ethernet port is assumed to be on a different subnet from the Wi-Fi (so that the strong and weak end-system models of 7.9.5   ARP and multihomed hosts do not play a role here). Assume A automatically selects the new Ethernet connection as its default route, with router RE. What happens to the original connection to A? Can packets still travel back and forth? Does the return address used for either direction change?
(b). You now disconnect A’s Wi-Fi interface, leaving the Ethernet interface connected. What happens now to the connection to B? Hint: to what IP address are the packets from B being sent?

See also 9   Routing-Update Algorithms, 9   Routing-Update Algorithms exercise 13.0, and 12   TCP Transport, exercise 13.0.