Your network guy might have a good reason for not wanting to share the information you enquired about. You see, what you describe you asked him of is not the IP range (CIDR) your company has been assigned to, but actual list of individual live IPs within that ASN. Now, getting the CIDR range that your organisation was assigned to its ASN is relatively easy, provided you know of a single IP belonging to that organisation and then simply search through all the registered ASNs. Alternatively, even searching for organisation's name might yield relevant ASNs it uses - see for example my answer for the How to get info on company , company owned sites etc… question. Much of this data is public knowledge.
That said, I believe you would get a completely different response, if you asked him of the CIDR range your company was assigned to, which is hardly a secret but probably good enough to assign new rules to a firewall. Sharing actual list of live IPs is a different matter tho;
Imagine your company has a
/24 range of public IP addresses (CIDR) assigned to its network (its ASN). That's only 256 IPs and quite common for smaller organisations. Now, let's assume your organisation really only uses half of these IPs, and the other IPs are dead. How could keeping this a secret serve your network guy? Let's assume an attacker wants to scan the CIDR range your organisation uses to collect server banners (software, hardware, operating system's identifying information or signatures) and enumerate live IPs. That's usually among the first steps an attacker would make. He'll go through the list of all IPs in your CIDR range, leaving fingerprints in access logs of various networking devices your network guy's setup. Your network guy can then easily determine what external IP was scanning your network's whole CIDR range worth of IP addresses (an attacker would scan all of them, including dead ones, while a legitimate user would only be accessing live ones) and promptly blacklist attackers IP to protect the network. Easy. If however the list of live IPs somehow became public (i.e. shared with an untrusted 3rd party), the chances an attacker creates this obvious attempt trail quickly diminish.
I think you should go back to your network guy and rather ask him for your public IP range next time. He shouldn't have much problems sharing that and is sufficient information 3rd parties need to know to whitelist your IPs, or otherwise identify access through them for other purposes, like e.g. automated client identification for extranet B2B services and alike.