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Free A-Level Computer Science guide
An address alone does not tell a host whether a destination is local. Apply the configured mask, then separate routing from name lookup and address translation.
Content owner: Michael Print · Written for A-Level learners · Checked against official specifications
Explore the idea
Compare two IPv4 addresses under the same prefix, follow a DHCP exchange, then inspect how a NAT mapping differs from a port-forwarding rule.
A subnet mask selects the network bits of an address. AND both addresses with the mask; equal network identifiers indicate the same configured subnet in the simple single-interface model. A remote destination is sent to a gateway, retaining the remote destination IP.
DHCP supplies time-limited address configuration. NAT translates addresses, often with ports; port forwarding supplies a particular inbound mapping. DNS resolves names. These are different operations and none alone establishes application trust or encryption.
AQA 7517 · 4.9.4.3–4.9.4.9
Useful foundations
By the end, you should be able to
AQA: network/host bits, masking, IPv4/IPv6, private/public addressing, DHCP, NAT and port forwarding. The subnet activity assumes one configured interface/default gateway; it does not model a complete router.
Shared foundations and optional extensions are labelled separately; this page does not claim complete coverage of the qualification.
An IPv4 address has 32 bits, usually written as four decimal octets. A /26 prefix fixes the first 26 network bits; its mask is 255.255.255.192. The last-octet mask is 11000000₂, giving blocks of 64 addresses.
192.168.10.65 AND that mask yields network 192.168.10.64. The range runs from .64 to .127. Under ordinary /26 host conventions, .64 identifies the subnet and .127 is its directed broadcast, leaving .65 through .126 for hosts.
A source configured as 192.168.10.65/26 treats 192.168.10.126 as local but .129 as remote. For local IPv4 delivery it resolves the destination's link-layer address; for remote delivery it resolves the next-hop gateway's link-layer address.
Routers forward packets using destination-network information. A routing table may have several matching prefixes, with the most specific applicable route chosen. This guide's activity assumes one subnet and one default gateway, without additional routes, VLANs or policy rules.
IPv6 has 128-bit addresses, written in hexadecimal groups. 2001:db8:0:0:0:0:0:5 may be shortened to 2001:db8::5; a double colon replaces one run of zero groups. The expanded space addresses IPv4 allocation pressure.
IPv4 private ranges include 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. They are not globally routed as public destinations. A public address is not automatically safe, and IPv6 has its own scopes and configuration mechanisms rather than requiring IPv4-style NAT everywhere.
A client discovers a server, receives an offer, requests the configuration and receives an acknowledgement. The lease can include an address, mask, gateway and DNS-server information. An offer alone is not a confirmed allocation.
A dynamic lease is time-limited and may be renewed. A statically configured address is assigned manually or through another administrative arrangement; it still must fit network configuration and avoid conflicts. DHCP is not the service that translates a domain name into an IP address.
An outbound connection from 192.168.1.8:51000 might appear publicly as 203.0.113.5:40001. A matching return packet is translated back using that flow mapping. These example public addresses are reserved documentation addresses.
An explicit forwarding rule could map public TCP 8443 to a private server's TCP 443. That inbound rule differs from a client's outbound mapping. A firewall can still reject the traffic; NAT alone neither encrypts it nor verifies the remote application. NAT/forwarding are explicit AQA depth; Cambridge learners should follow their syllabus's named network requirements.
Worked example
4 original questions total 14 marks. Attempt each before opening the independently written indicative marking guidance.
For source 192.168.4.130/26, give the mask, network address, directed broadcast and ordinary host range. Is 192.168.4.190 local under that mask?
Indicative answer
Name the four allocation messages in a basic DHCP exchange and distinguish DHCP from DNS.
Indicative answer
AQA depth: an outbound client maps private 192.168.1.8:51000 to public 203.0.113.5:40001. Explain the return mapping and why it is not a general inbound server-forwarding rule.
Indicative answer
State the bit lengths of IPv4 and IPv6, expand 2001:db8::5 fully, and explain one reason for IPv6.
Indicative answer
This guide addresses AQA 7517 4.9.4.3–4.9.4.9. Check your examination year and the complete specification for the assessment scope.
These are independently written explanations and practice questions. CompSciTutoring.co.uk is not affiliated with or endorsed by an examination board. The marking guidance is indicative; always check the syllabus for your examination year.
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