Many DNS administrators treat apex NS records as mere formalities, but their actual function is more nuanced than commonly believed. While subdomain NS records clearly signal delegation (like sub1 IN NS ns1.example.edu. in your example), apex NS records serve three distinct technical purposes:

; Example zone file snippet showing proper apex NS usage
$ORIGIN example.com.
@ 3600 IN NS ns1.example.com.
@ 3600 IN NS ns2.example.com.
ns1 3600 IN A 198.51.100.2  ; Glue record
ns2 3600 IN A 198.51.100.3  ; Glue record

Modern DNS software handles apex NS records differently than subdomain delegations. Here's what happens under the hood in BIND 9:

zone "example.com" {
    type master;
    file "/etc/bind/db.example.com";
    // These NS records DO NOT trigger delegation
    // but enable important secondary functions
};

While glue records at the registrar level provide initial authority information, apex NS records enable critical DNSSEC validation:

1. Validating resolvers cross-check NS records against DS records
2. Mismatches trigger SERVFAIL responses (RFC 4035 Section 5)
3. Example validation failure scenario:

; Registrar glue shows:
example.com. NS ns1.otherprovider.net

; Zone file shows:
@ IN NS ns1.example.com.  ; Mismatch!

For optimal DNS hygiene:

• Keep apex NS records synchronized with registrar glue
• Use consistent TTL values (3600 seconds recommended)
• Include all authoritative servers, not just primaries

# Diagnostic command to verify consistency
dig +norec @a.gtld-servers.net example.com NS
dig +trace example.com SOA

While NS records in subdomains clearly serve delegation purposes, their role at the zone apex (e.g., example.com.) is often misunderstood. The zone file snippet below illustrates a typical configuration:

$ORIGIN example.com.
@        IN     NS   ns1.example.com.
@        IN     NS   ns2.example.com.

Many developers incorrectly assume these records:

• Directly control glue records (they don't - glue is registered separately)
• Facilitate domain-wide delegation (they don't - zone authority is configured separately in DNS software)
• Determine authoritative responses (they don't - the AA flag comes from zone configuration)

Apex NS records serve these critical functions:

; Zone integrity verification
$ dig +norec @8.8.8.8 example.com NS

; DNSSEC chain of trust
example.com. 3600 IN NSEC  ns1.example.com. ( ... )

They provide:

1. A self-reference mechanism for DNS servers when walking the delegation chain
2. Required metadata for DNSSEC validation
3. The official list of nameservers that should be authoritative for the zone

For proper configuration:

; Correct:
@        IN NS ns1.provider.com.
@        IN NS ns2.provider.com.

; Problematic (common mistake):
@        IN NS ns1.example.com.  ; Requires glue to resolve
@        IN NS ns2.example.com.  ; Circular if not properly registered

Key considerations:

• Always match registrar-glued nameservers
• Include at least 2 distinct NS records
• Use FQDNs (trailing dots) for absolute references

Here's how major providers implement this:

; Cloudflare example
example.com.  86400  IN  NS  leia.ns.cloudflare.com.
example.com.  86400  IN  NS  luke.ns.cloudflare.com.

; AWS Route 53
example.com.  172800 IN  NS  ns-123.awsdns-15.com.

The consistency across providers demonstrates this isn't arbitrary configuration but part of fundamental DNS architecture.

Watch for these warning signs:

; Mismatch between glue and zone NS
$ dig +trace example.com NS | grep "example.com. IN NS"

; LAME delegation errors in nameserver logs
named[123]: lame server resolving 'example.com' (in 'example.com'?): 1.2.3.4#53

Always verify using multiple tools like dig, dnsviz, and registrar DNS checks.